QGIS User Guide Manual 2.8

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QGIS User Guide
Publicación 2.8
QGIS Project
07 de August de 2015
Contents
1 Preámbulo 3
2 Convenciones 5
2.1 Convenciones de la Interfaz Gráfica o GUI ............................. 5
2.2 Convenciones de Texto o Teclado .................................. 5
2.3 Instrucciones específicas de cada plataforma ............................ 6
3 Prólogo 7
4 Características 9
4.1 Ver datos ............................................... 9
4.2 Explorar datos y componer mapas .................................. 9
4.3 Crear, editar, gestionar y exportar datos ............................... 10
4.4 Analizar datos ............................................ 10
4.5 Publicar mapas en Internet ...................................... 10
4.6 Extend QGIS functionality through plugins ............................. 10
4.7 Consola de Python .......................................... 11
4.8 Problemas Conocidos ........................................ 12
5 Qué es lo nuevo en QGIS 2.8 13
5.1 Application .............................................. 13
5.2 Proveedor de datos .......................................... 13
5.3 Digitizing ............................................... 14
5.4 Diseñador de impresión de Mapa .................................. 14
5.5 Plugins ................................................ 14
5.6 Servidor QGIS ............................................ 14
5.7 Simbología .............................................. 14
5.8 Interfaz de Usuario .......................................... 14
6 Comenzar 15
6.1 Instalación .............................................. 15
6.2 Datos de ejemplo ........................................... 15
6.3 Sesión de ejemplo .......................................... 16
6.4 Iniciar y cerrar QGIS ......................................... 17
6.5 Opciones de la línea de órdenes ................................... 17
6.6 Proyectos ............................................... 19
6.7 Salida ................................................. 20
7 QGIS GUI 21
7.1 Barra de Menú ............................................ 22
7.2 Barra de herramietas ......................................... 29
7.3 Leyenda del mapa .......................................... 29
7.4 Vista del mapa ............................................ 32
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7.5 Barra de Estado ............................................ 32
8 Herramientas generales 35
8.1 Teclas de acceso rápido ....................................... 35
8.2 Ayuda de contexto .......................................... 35
8.3 Renderizado ............................................. 35
8.4 Mediciones .............................................. 37
8.5 Identificar objetos espaciales ..................................... 39
8.6 Elementos decorativos ........................................ 40
8.7 Herramientas de anotaciones ..................................... 43
8.8 Marcadores espaciales ........................................ 44
8.9 Anidar proyectos ........................................... 45
9 Configuración QGIS 47
9.1 Paneles y Barras de Herramientas .................................. 47
9.2 Propiedades del proyecto ....................................... 48
9.3 Opciones ............................................... 49
9.4 Personalización ............................................ 57
10 Working with Projections 59
10.1 Overview of Projection Support ................................... 59
10.2 Global Projection Specification ................................... 59
10.3 Define On The Fly (OTF) Reprojection ............................... 61
10.4 Custom Coordinate Reference System ................................ 62
10.5 Default datum transformations .................................... 63
11 QGIS Browser 65
12 Trabajar con catos vectoriales 67
12.1 Supported Data Formats ....................................... 67
12.2 The Symbol Library ......................................... 79
12.3 The Vector Properties Dialog .................................... 82
12.4 Expressions ..............................................112
12.5 Editing ................................................118
12.6 Constructor de consultas .......................................135
12.7 Field Calculator ...........................................136
13 Trabajar con catos raster 139
13.1 Working with Raster Data ......................................139
13.2 Raster Properties Dialog .......................................140
13.3 Calculadora Ráster ..........................................148
14 Trabajar con datos OGC 151
14.1 QGIS como cliente de datos OGC ..................................151
14.2 QGIS como Servidor de Datos OGC .................................161
15 Trabajar con datos GPS 167
15.1 GPS Plugin ..............................................167
15.2 Live GPS tracking ..........................................171
16 GRASS GIS Integration 177
16.1 Starting the GRASS plugin .....................................177
16.2 Loading GRASS raster and vector layers ..............................178
16.3 GRASS LOCATION and MAPSET .................................178
16.4 Importing data into a GRASS LOCATION .............................180
16.5 The GRASS vector data model ...................................181
16.6 Creating a new GRASS vector layer .................................182
16.7 Digitizing and editing a GRASS vector layer ............................182
16.8 The GRASS region tool .......................................185
16.9 The GRASS Toolbox .........................................185
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17 Entorno de trabajo de procesamiento de QGIS 195
17.1 Introducción .............................................195
17.2 La caja de herramientas .......................................196
17.3 &Modelador gráfico... ........................................205
17.4 La interfaz de procesamiento por lotes ................................211
17.5 Utilizar algoritmos de procesamiento desde la consola .......................213
17.6 El administrador del historial ....................................218
17.7 Writing new Processing algorithms as python scripts ........................219
17.8 Handing data produced by the algorithm ..............................221
17.9 Communicating with the user ....................................221
17.10 Documenting your scripts ......................................221
17.11 Example scripts ...........................................222
17.12 Best practices for writing script algorithms .............................222
17.13 Pre- and post-execution script hooks .................................222
17.14 Configurar aplicaciones externas ...................................222
17.15 Los Comandos QGIS .........................................229
18 Diseñadores de impresión 231
18.1 Primeros pasos ............................................233
18.2 Modo de representación .......................................237
18.3 Elementos de diseño .........................................238
18.4 Administrar elementos ........................................261
18.5 Revertir y Restaurar herramientas ..................................263
18.6 Generación de Atlas .........................................263
18.7 Ocultar y mostrar paneles ......................................265
18.8 Crear salida ..............................................266
18.9 Administrar el diseñador de impresión ................................267
19 Complementos 269
19.1 QGIS Complementos .........................................269
19.2 Usar complementos núcleo de QGIS ................................275
19.3 Complemento Captura de coordenadas ...............................276
19.4 Complemento administrador de BBDD ...............................276
19.5 Complemento Conversor DxfaShp ..................................278
19.6 Complemento Visualización de Eventos ...............................279
19.7 Complemento fTools .........................................289
19.8 Complemento Herramientas de GDAL ...............................293
19.9 Complemento Georreferenciador ..................................297
19.10 Complemento Mapa de calor .....................................302
19.11 Complemento de interpolación ....................................305
19.12 MetaSearch Catalogue Client ....................................307
19.13 Complemento Edición fuera de linea ................................310
19.14 Complemento GeoRaster espacial de Oracle ............................311
19.15 Complemento Análisis de Terreno ..................................313
19.16 Complemento Grafo de rutas .....................................314
19.17 Complemento Consulta espacial ...................................315
19.18 Complemento SPIT .........................................317
19.19 Complemento Comprobador de topología. .............................317
19.20 Complemento de Estadísticas de zona ................................320
20 Ayuda y apoyo 321
20.1 Listas de correos ...........................................321
20.2 IRC ..................................................322
20.3 Rastreador de Errores ........................................322
20.4 Blog .................................................323
20.5 Plugins ................................................323
20.6 Wiki .................................................323
21 Apéndice 325
iii
21.1 GNU General Public License ....................................325
21.2 GNU Free Documentation License .................................328
22 Referencias bibliográficas y web 335
Índice 337
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QGIS User Guide, Publicación 2.8
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Contents 1
QGIS User Guide, Publicación 2.8
2 Contents
CHAPTER 1
Preámbulo
Este documento es la guía de usuario original del software QGIS que se describe. El software y el hardware
descritos en este documento son el la mayoría de los casos marcas registradas y por lo tanto están sujetos a
requisitos legales. QGIS está sujeto a la Licencia Pública General GNU. Encontrará más información en la página
de QGIS, http://www.qgis.org.
Los detalles, datos y resultados en este documento han sido escritos y verificados con el mejor de los conocimien-
tos y responsabilidad de los autores y editores. Sin embargo, son posibles errores en el contenido.
Por lo tanto, los datos no están sujetos a ningún derecho o garantía. Los autores y editores no aceptan ninguna
responsabilidad u obligación por fallos y sus consecuencias. Siempre será bienvenido a informar posibles errores.
Este documento ha sido compuesto con reStructuredText. Está disponible como código fuente reST vía github y
en línea como HTML y PDF en http://www.qgis.org/en/docs/. También se pueden descargar versiones traducidas
de este documento en varios formatos en el área de documentación del proyecto QGIS. Para mayor información
sobre contribuir a este documento y acerca de la traducción, por favor visite http://www.qgis.org/wiki/.
Enlaces en este documento
Este documento contiene enlaces internos y externos. Pulsando un enlace interno navega dentro del documento,
mientras que pulsando un enlace externo abre una dirección de Internet. En formato PDF, los enlaces internos y
externos son mostrados en azul y son manejados por el navegador del sistema. En formato HTML, el navegador
muestra y maneja ambos de manera idéntica.
Autores y Editores de las Guías de Usuario, Instalación y Programación:
Tara Athan Radim Blazek Godofredo Contreras Otto Dassau Martin Dobias
Peter Ersts Anne Ghisla Stephan Holl N. Horning Magnus Homann
Werner Macho Carson J.Q. Farmer Tyler Mitchell K. Koy Lars Luthman
Claudia A. Engel Brendan Morely David Willis Jürgen E. Fischer Marco Hugentobler
Larissa Junek Diethard Jansen Paolo Corti Gavin Macaulay Gary E. Sherman
Tim Sutton Alex Bruy Raymond Nijssen Richard Duivenvoorde Andreas Neumann
Astrid Emde Yves Jacolin Alexandre Neto Andy Schmid Hien Tran-Quang
Copyright (c) 2004 - 2014 Equipo de desarrollo de QGIS
Internet: http://www.qgis.org
Licencia de este documento
Se permite la copia, distribución y/o modificación de este documento bajo los términos de la Licencia de Docu-
mentación Libre GNU, Versión 1.3 o cualquier versión posterior publicada por la Fundación de Software Libre;
sin Secciones Invariante, ni Texto de Portada ni de Contracubierta. Se incluye una copia de la licencia en el
Apéndice GNU Free Documentation License.
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4 Chapter 1. Preámbulo
CHAPTER 2
Convenciones
Esta sección describe los estilos homogéneos que se utilizarán a lo largo de este manual.
2.1 Convenciones de la Interfaz Gráfica o GUI
Las convenciones de estilo del GUI están destinadas a imitar la apariencia de la interfaz gráfica de usuario. En
general, un estilo reflejará la apariencia simplificada, por lo que un usuario puede escanear visualmente el GUI
para encontrar algo que se parece a lo mostrado en el manual.
Menú Opciones: Capa Añadir capa ráster oPreferencias Barra de Herramientas Digitalizacion
• Herramienta: Añadir capa ráster
Boton : [Guardar como]
Título del Cuadro de Diálogo: Propiedades de capa
Pestaña: General
• Selección: Renderizar
Botón de selección: Postgis SRID EPSG ID
Seleccionar un número:
Seleccionar una cadena:
Buscar un archivo:
Seleccione un color:
Barra de desplazamiento:
Texto de Entrada:
El sombreado muestra un componente de la interfaz que el usuario puede pulsar.
2.2 Convenciones de Texto o Teclado
Este manual también incluye estilos relacionadas con el texto, los comandos de teclado y codificación para indicar
diferentes entidades, como las clases o métodos. Estos estilos no se corresponden con la apariencia real de
cualquier texto o codificación dentro QGIS.
Hiperenlaces: http://qgis.org
Combinaciones de Teclas: Pulsar Ctrl+B, significa mantener pulsada la tecla Ctrl y pulsar la letra B.
Nombre de un Archivo: lakes.shp
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QGIS User Guide, Publicación 2.8
Nombre de una Clase: NewLayer
Método: classFactory
Servidor: myhost.de
Texto para el Usuario: qgis --help
Las líneas de código se muestran con una fuente de ancho fijo:
PROJCS["NAD_1927_Albers",
GEOGCS["GCS_North_American_1927",
2.3 Instrucciones específicas de cada plataforma
Algunas secuencias GUI y pequeñas cantidades de texto pueden ser formateados en línea : Haga clic
:menuselection: Archivo QGIS Salir para cerrar QGIS. Esto indica que en Linux , Unix y plataformas
Windows, debe hacer clic en el menú Archivo, y luego en Salir, mientras que en Macintosh OS X, debe hacer clic
en el Menú QGIS primero, y luego en Salir.
Las cantidades mayores de texto se pueden formatear como listas:
Hacer esto
Hacer aquello
Hacer otra cosa
o como párrafos:
Hacer esto y esto y esto. Entonces hacer esto y esto y esto, y esto y esto y esto, y esto y esto y esto.
Hacer eso. Entonces hacer eso y eso y eso, y eso y eso y eso y eso, y eso y eso y eso, y eso y eso y eso, y eso y
eso y eso.
Las capturas de pantalls que aparecen a lo largo de la guía de usuario han sido creadas en diferentes plataformas;
éstas se indicarán por el icono específico para cada una al final del pie de imagen.
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6 Chapter 2. Convenciones
CHAPTER 3
Prólogo
¡Bienvenido al maravilloso mundo de los Sistemas de Información Geográfica (SIG)!
QGIS es un Sistema de Información Geográfica de código abierto. El proyecto nació en mayo de 2002 y se
estableció como un proyecto en SourceForge en junio del mismo año. Hemos trabajado duro para hacer que el
software SIG (tradicionalmente software propietario caro) esté al alcance de cualquiera con acceso básico a un
ordenador personal. QGIS actualmente funciona en la mayoría de plataformas Unix, Windows y OS X. QGIS se
desarrolla usando el kit de herramientas Qt (http://qt.digia.com) y C++. Esto significa que QGIS es ligero y tiene
una interfaz gráfica de usuario (GUI) agradable y fácil de usar.
QGIS pretende ser un SIG amigable, proporcionando funciones y características comunes. El objetivo inicial
del proyecto era proporcionar un visor de datos SIG. QGIS ha alcanzado un punto en su evolución en el que
está siendo usado por muchos para sus necesidades diarias de visualización de datos SIG. QGIS admite diversos
formatos de datos ráster y vectoriales, pudiendo añadir nuevos formatos usando la arquitectura de complementos.
QGIS se distribuye bajo la Licencia Pública General GNU (GPL). El desarrollo de QGIS bajo esta licencia sig-
nifica que se puede revisar y modificar el código fuente y garantiza que usted, nuestro feliz usuario, siempre
tendrá acceso a un programa de SIG que es libre de costo y puede ser libremente modificado. Debería haber
recibido una copia completa de la licencia con su copia de QGIS, y también podrá encontrarla en el Apéndice
:ref:gpl_appendix.
Truco: Documentación al día
La última versión de este documento siempre se puede encontrar en el área de documentación de la web de QGIS
en http://www.qgis.org/en/docs/.
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8 Chapter 3. Prólogo
CHAPTER 4
Características
QGIS offers many common GIS functionalities provided by core features and plugins. A short summary of six
general categories of features and plugins is presented below, followed by first insights into the integrated Python
console.
4.1 Ver datos
You can view and overlay vector and raster data in different formats and projections without conversion to an
internal or common format. Supported formats include:
Spatially-enabled tables and views using PostGIS, SpatiaLite and MS SQL Spatial, Oracle Spatial, vector
formats supported by the installed OGR library, including ESRI shapefiles, MapInfo, SDTS, GML and
many more. See section Trabajar con catos vectoriales.
Raster and imagery formats supported by the installed GDAL (Geospatial Data Abstraction Library) library,
such as GeoTIFF, ERDAS IMG, ArcInfo ASCII GRID, JPEG, PNG and many more. See section Trabajar
con catos raster.
GRASS raster and vector data from GRASS databases (location/mapset). See section GRASS GIS Integra-
tion.
Online spatial data served as OGC Web Services, including WMS, WMTS, WCS, WFS, and WFS-T. See
section Trabajar con datos OGC.
4.2 Explorar datos y componer mapas
You can compose maps and interactively explore spatial data with a friendly GUI. The many helpful tools available
in the GUI include:
Explorador QGIS
Reproyección al vuelo
Gestor de Base de Datos
Diseñador de mapas
Panel de vista general
Marcadores espaciales
Herramientas de anotaciones
Identify/select features
Editar/ver/buscar atributos
Data-defined feature labeling
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Data-defined vector and raster symbology tools
Atlas map composition with graticule layers
North arrow scale bar and copyright label for maps
Support for saving and restoring projects
4.3 Crear, editar, gestionar y exportar datos
You can create, edit, manage and export vector and raster layers in several formats. QGIS offers the following:
Digitizing tools for OGR-supported formats and GRASS vector layers
Ability to create and edit shapefiles and GRASS vector layers
Georeferencer plugin to geocode images
GPS tools to import and export GPX format, and convert other GPS formats to GPX or down/upload directly
to a GPS unit (On Linux, usb: has been added to list of GPS devices.)
Support for visualizing and editing OpenStreetMap data
Ability to create spatial database tables from shapefiles with DB Manager plugin
Improved handling of spatial database tables
Tools for managing vector attribute tables
Option to save screenshots as georeferenced images
DXF-Export tool with enhanced capabilities to export styles and plugins to perform CAD-like functions
4.4 Analizar datos
You can perform spatial data analysis on spatial databases and other OGR- supported formats. QGIS currently
offers vector analysis, sampling, geoprocessing, geometry and database management tools. You can also use the
integrated GRASS tools, which include the complete GRASS functionality of more than 400 modules. (See sec-
tion GRASS GIS Integration.) Or, you can work with the Processing Plugin, which provides a powerful geospatial
analysis framework to call native and third-party algorithms from QGIS, such as GDAL, SAGA, GRASS, fTools
and more. (See section Introducción.)
4.5 Publicar mapas en Internet
QGIS can be used as a WMS, WMTS, WMS-C or WFS and WFS-T client, and as a WMS, WCS or WFS server.
(See section Trabajar con datos OGC.) Additionally, you can publish your data on the Internet using a webserver
with UMN MapServer or GeoServer installed.
4.6 Extend QGIS functionality through plugins
QGIS can be adapted to your special needs with the extensible plugin architecture and libraries that can be used
to create plugins. You can even create new applications with C++ or Python!
10 Chapter 4. Características
QGIS User Guide, Publicación 2.8
4.6.1 Complementos del Núcleo
Los complementos del núcleo incluyen:
1. Coordinate Capture (Capture mouse coordinates in different CRSs)
2. DB Manager (Exchange, edit and view layers and tables; execute SQL queries)
3. Dxf2Shp Converter (Convert DXF files to shapefiles)
4. eVIS (Visualizar eventos)
5. fTools (Analyze and manage vector data)
6. GDALTools (Integrate GDAL Tools into QGIS)
7. Georeferencer GDAL (Add projection information to rasters using GDAL)
8. Herramientas GPS (cargar e importar datos de GPS)
9. GRASS (integrar el SIG GRASS)
10. Heatmap (Generate raster heatmaps from point data)
11. Interpolation Plugin (Interpolate based on vertices of a vector layer)
12. Metasearch Catalogue Client
13. Offline Editing (Allow offline editing and synchronizing with databases)
14. GeoRaster Espacial de Oracle
15. Procesamiento (antiguamente SEXTANTE)
16. Raster Terrain Analysis (Analyze raster-based terrain)
17. Road Graph Plugin (Analyze a shortest-path network)
18. Complemento de consulta espacial
19. SPIT (Import shapefiles to PostgreSQL/PostGIS)
20. Topology Checker (Find topological errors in vector layers)
21. Zonal Statistics Plugin (Calculate count, sum, and mean of a raster for each polygon of a vector layer)
4.6.2 Complementos externos de Python
QGIS offers a growing number of external Python plugins that are provided by the community. These plugins
reside in the official Plugins Repository and can be easily installed using the Python Plugin Installer. See Section
El diálogo de complementos.
4.7 Consola de Python
For scripting, it is possible to take advantage of an integrated Python console, which can be opened from menu:
Plugins Python Console. The console opens as a non-modal utility window. For interaction with the QGIS en-
vironment, there is the qgis.utils.iface variable, which is an instance of QgsInterface. This interface
allows access to the map canvas, menus, toolbars and other parts of the QGIS application. You can create a script,
then drag and drop it into the QGIS window and it will be executed automatically.
For further information about working with the Python console and programming QGIS plugins and applications,
please refer to PyQGIS-Developer-Cookbook.
4.7. Consola de Python 11
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4.8 Problemas Conocidos
4.8.1 Limitación en el número de archivos abiertos
If you are opening a large QGIS project and you are sure that all layers are valid, but some layers are flagged as
bad, you are probably faced with this issue. Linux (and other OSs, likewise) has a limit of opened files by process.
Resource limits are per-process and inherited. The ulimit command, which is a shell built-in, changes the limits
only for the current shell process; the new limit will be inherited by any child processes.
You can see all current ulimit info by typing
user@host:~$ ulimit -aS
You can see the current allowed number of opened files per proccess with the following command on a console
user@host:~$ ulimit -Sn
To change the limits for an existing session, you may be able to use something like
user@host:~$ ulimit -Sn #number_of_allowed_open_files
user@host:~$ ulimit -Sn
user@host:~$ qgis
To fix it forever
On most Linux systems, resource limits are set on login by the pam_limits module according to the settings
contained in /etc/security/limits.conf or /etc/security/limits.d/*.conf. You should be
able to edit those files if you have root privilege (also via sudo), but you will need to log in again before any
changes take effect.
Más información:
http://www.cyberciti.biz/faq/linux-increase-the-maximum-number-of-open-files/ http://linuxaria.com/article/open-
files-in-linux?lang=en
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12 Chapter 4. Características
CHAPTER 5
Qué es lo nuevo en QGIS 2.8
Esta versión contiene nuevas características y se extiende la interfaz de programación con respecto a versiones
anteriores. Le recomendamos que utilice esta versión sobre las versiones anteriores.
This release includes hundreds of bug fixes and many new features and enhancements
that will be described in this manual. You may also review the visual changelog at
http://qgis.org/en/site/forusers/visualchangelog28/index.html.
5.1 Application
Map rotation: A map rotation can be set in degrees from the status bar
Bookmarks: You can share and transfer your bookmarks
Expressions:
when editing attributes in the attribute table or forms, you can now enter expressions directly into spin
boxes
the expression widget is extended to include a function editor where you are able to create your own
Python custom functions in a comfortable way
in any spinbox of the style menu you can enter expressions and evaluate them immediately
a get and transform geometry function was added for using expressions
a comment functionality was inserted if for example you want to work with data defined labeling
Joins: You can specify a custom prefix for joins
Layer Legend: Show rule-based renderer’s legend as a tree
DB Manager: Run only the selected part of a SQL query
Attribute Table: support for calculations on selected rows through a ‘Update Selected’ button
Measure Tools: change measurement units possible
5.2 Proveedor de datos
DXF Export tool improvements: Improved marker symbol export
WMS Layers: Support for contextual WMS legend graphics
Temporary Scratch Layers: It is possible to create empty editable memory layers
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5.3 Digitizing
Advanced Digitizing:
digitise lines exactly parallel or at right angles, lock lines to specific angles and so on with the advanced
digitizing panel (CAD-like features)
simplify tool: specify with exact tolerance, simplify multiple features at once ...
Snapping Options: new snapping mode ‘Snap to all layers’
5.4 Diseñador de impresión de Mapa
Composer GUI improvements: hide bounding boxes, full screen mode for composer toggle display of
panels
Grid improvements: You now have finer control of frame and annotation display
Label item margins: You can now control both horizontal and vertical margins for label items. You can
now specify negative margins for label items.
optionally store layer styles
Attribute Table Item: options ‘Current atlas feature’ and ‘Relation children’ in Main properties
5.5 Plugins
Python Console: You can now drag and drop python scripts into the QGIS window
5.6 Servidor QGIS
Python plugin support
5.7 Simbología
live heatmap renderer creates dynamic heatmaps from point layers
raster image symbol fill type
more data-defined symbology settings: the data-defined option was moved next to each data definable
property
support for multiple styles per map layer, optionally store layer styles
5.8 Interfaz de Usuario
Projection: Improved/consistent projection selection. All dialogs now use a consistent projection selection
widget, which allows for quickly selecting from recently used and standard project/QGIS projections
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14 Chapter 5. Qué es lo nuevo en QGIS 2.8
CHAPTER 6
Comenzar
Este capítulo da una vista general rápida sobre la instalación de QGIS, algunos datos de ejemplo de la web de
QGIS y ejecutar una primera sesión sencilla visualizando capas ráster y vectoriales.
6.1 Instalación
La instalación de QGIS es muy sencilla. Hay disponibles paquetes de instalación estándar para MS Windows y
Mac OS X. Se proporcionan paquetes binarios (rpm y deb) o repositorios de software para añadir a su gestor de
paquetes para muchos sabores de GNU/Linux. Consiga la última información sobre paquetes binarios en la web
de QGIS en http://download.qgis.org.
6.1.1 Instalación a partir de las fuentes
Si necesita compilar QGIS a partir de las fuentes, por favor consulte las instrucciones de instalación. Se dis-
tribuyen con el código fuente de QGIS en un archivo llamado INSTALL. También puede encontrarlas en línea en
http://htmlpreview.github.io/?https://raw.github.com/qgis/QGIS/master/doc/INSTALL.html
6.1.2 Instalación en medios extraíbles
QGIS le permite definir una opción --configpath que suplanta la ruta predeterminada para la configuración
de usuario (ej.: ~/.qgis2 bajo Linux) y fuerza a QSettings a usar ese directorio. Esto le permite, por ejemplo,
llevar una instalación de QGIS en una memoria flash junto con todos los complementos y la configuración. Vea la
sección Menú Sistema para información adicional.
6.2 Datos de ejemplo
La guía de usuario contiene ejemplos basados en el conjunto de datos de ejemplo de QGIS.
El instalador de Windows tiene una opción para descargar el conjunto de datos de muestra de QGIS. Si se
marca, los datos se decargarán en su carpeta Mis Documentos y se colocarán en una carpeta llamada GIS
Database. Puede usar el Explorador de Windows para mover esta carpeta a una ubicación adecuada. Si no
marcó la casilla de verificación para instalar el conjunto de datos de muestra durante la instalación inicial de
QGIS, puede hacer algo de lo siguiente:
Usar datos SIG que ya tenga
Download sample data from http://qgis.org/downloads/data/qgis_sample_data.zip
Desinstalar QGIS y volver a instalarlo con la opción de descarga de datos marcada (sólo recomendado si las
soluciones anteriores no funcionaron).
15
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For GNU/Linux and Mac OS X, there are not yet dataset installation packages available as rpm,
deb or dmg. To use the sample dataset, download the file qgis_sample_data as a ZIP archive from
http://qgis.org/downloads/data and unzip the archive on your system.
El conjunto de datos de Alaska incluye todos los datos SIG que se usan para los ejemplos y capturas de pantalla
de la guía de usuario; también incluye una pequeña base de datos de GRASS. La proyección del conjunto de datos
de QGIS es Alaska Albers Equal Area con unidades en pies. El código EPSG es 2964.
PROJCS["Albers Equal Area",
GEOGCS["NAD27",
DATUM["North_American_Datum_1927",
SPHEROID["Clarke 1866",6378206.4,294.978698213898,
AUTHORITY["EPSG","7008"]],
TOWGS84[-3,142,183,0,0,0,0],
AUTHORITY["EPSG","6267"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.0174532925199433,
AUTHORITY["EPSG","9108"]],
AUTHORITY["EPSG","4267"]],
PROJECTION["Albers_Conic_Equal_Area"],
PARAMETER["standard_parallel_1",55],
PARAMETER["standard_parallel_2",65],
PARAMETER["latitude_of_center",50],
PARAMETER["longitude_of_center",-154],
PARAMETER["false_easting",0],
PARAMETER["false_northing",0],
UNIT["us_survey_feet",0.3048006096012192]]
Si pretende usar QGIS como un visor gráfico para GRASS, puede encontrar una selección de lo-
calizaciones de ejemplo (ej.., Spearfish o Dakota de Sur) en la web oficial de GRASS GIS,
http://grass.osgeo.org/download/sample-data/.
6.3 Sesión de ejemplo
Ahora que tiene QGIS instalado y un dispone de un conjunto de datos, nos gustaría mostrarle una sesión
de muestra de QGIS corta y sencilla. Visualizaremos una capa ráster y otra vectorial. Usaremos la
capa ráster landcover,qgis_sample_data/raster/landcover.img y la capa vectorial lakes,
qgis_sample_data/gml/lakes.gml.
6.3.1 Iniciar QGIS
Arranque QGIS tecleando “QGIS” en la línea de órdenes o si usa un binario precompilado, usando el
menú Aplicaciones.
Iniciar QGIS usando el menú Inicio o accesos directos en el escritorio o haciendo doble clic en un archivo
de proyecto de QGIS.
Hacer doble clic en el icono de su carpeta Aplicaciones.
6.3.2 Cargar capas ráster y vectoriales del conjunto de datos de ejemplo
1. Haga clic en el icono Añadir capa ráster
2. Navegue a la carpeta qgis_sample_data/raster/, seleccione el archivo ERDAS IMG
landcover.img y haga clic en [Abrir].
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3. Si el archivo no está en la lista, compruebe si el listado Tipo de archivos en la parte inferior del cuadro
de diálogo se encuentra en el tipo correcto, en este caso “Erdas Imagine Images” (*.img, *.IMG)”.
4. Ahora haga clic en el icono Añadir capa vectorial.
5. Archivo debería estar seleccionado como Tipo de origien en el nuevo diálogo Añadir capa vectorial.
Ahora haga clic en [Explorar] para seleccionar la capa vectorial.
6. Navegue a la carpeta qgis_sample_data/gml/, y seleccionar ‘Geography Markup Language [GML]
[OGR] (.gml,.GML)’ de la lista desplegable Filtro , a continuación seleccionar el archivo GML
lakes.gml y haga clic [Abrir]. En el diálogo Añadir capa vectorial, haga clic en [Aceptar]. El diálogo
Selector de Sistema de Referencia de Coordenadas se abrirá con NAD27 / Alaska Alberts seleccionada, haga
clic en [Aceptar].
7. Acerque el zoom un poco a la zona que prefiera con algunos lagos.
8. Haga doble clic en la capa lakes en el panel Capas para abrir el diálogo Propiedades.
9. Clic en la pestaña Estilo y seleccionar un azul como color de relleno.
10. Haga clic en la pestaña Etiquetas‘y marque la casilla |checkbox| :guilabel:‘Etiquetar esta capa con para
habilitar el etiquetado. Seleccione el campo “NAMES” como el campo que contiene las etiquetas.
11. Para mejorar la lectura de las etiquetas, puede añadir una zona blanca a su alrederor haciendo clic en “Már-
gen” en la lista de la izquierda, marcando Dibujar buffer de texto y eligiendo 3 como tamaño de buffer.
12. Haga clik en [Aplicar]. Compruebe si el resultado le gusta y finalmente pulse [Aceptar].
Puede ver lo fácil que es visualizar capas ráster y vectoriales en QGIS. Vayamos a las secciones que siguen para
aprender más sobre las funcionalidades, características y configuración disponibles y cómo usarlas.
6.4 Iniciar y cerrar QGIS
En la sección Sesión de ejemplo ya aprendió como iniciar QGIS. Repetiremos esto aquí y verá que QGIS también
proporciona otras opciones de línea de órdenes.
Asumiendo que QGIS está instalado en el PATH, puede iniciar QGIS tecleando qgis en la consola
o haciendo doble clic en el enlace (o acceso directo) a la aplicación QGIS en el escritorio o en el menú
Aplicaciones.
Iniciar QGIS usando el menú Inicio o accesos directos en el escritorio o haciendo doble clic en un archivo
de proyecto de QGIS.
Haga doble clic en el icono en su carpeta Aplicaciones. Si necesita iniciar QGIS en una consola, ejecute
/path-to-installation-executable/Contents/MacOS/Qgis.
Para detener QGIS, haga clic en la opción de menú Archivo QGIS Salir, o use use el atajo Ctrl+Q.
6.5 Opciones de la línea de órdenes
QGIS admite diversas opciones cuando se arranca desde la línea de órdenes. Para obteter una lista de las
opciones, introduzca qgis --help en la línea de órdenes. La sentencia de uso para QGIS es:
qgis --help
QGIS - 2.6.0-Brighton ’Brighton’ (exported)
QGIS is a user friendly Open Source Geographic Information System.
Usage: /usr/bin/qgis.bin [OPTION] [FILE]
OPTION:
[--snapshot filename] emit snapshot of loaded datasets to given file
[--width width] width of snapshot to emit
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[--height height] height of snapshot to emit
[--lang language] use language for interface text
[--project projectfile] load the given QGIS project
[--extent xmin,ymin,xmax,ymax] set initial map extent
[--nologo] hide splash screen
[--noplugins] don’t restore plugins on startup
[--nocustomization] don’t apply GUI customization
[--customizationfile] use the given ini file as GUI customization
[--optionspath path] use the given QSettings path
[--configpath path] use the given path for all user configuration
[--code path] run the given python file on load
[--defaultui] start by resetting user ui settings to default
[--help] this text
FILE:
Files specified on the command line can include rasters,
vectors, and QGIS project files (.qgs):
1. Rasters - supported formats include GeoTiff, DEM
and others supported by GDAL
2. Vectors - supported formats include ESRI Shapefiles
and others supported by OGR and PostgreSQL layers using
the PostGIS extension
Truco: Ejemplo usando argumentos de la línea de órdenes
Puede iniciar QGIS especificando uno o más archivos de datos en la línea de órdenes. Por ejemplo, asumiendo
que está en el directorio qgis_sample_data, podría iniciar QGIS con una capa vectorial y un archivo ráster
establecidos para que se carguen al inicio usando la siguiente orden: qgis ./raster/landcover.img
./gml/lakes.gml
Opción de la línea de órdenes --snapshot
Esta opción permite crear una captura de pantalla en formato PNG de la vista actual. Esto es práctico cuando tiene
muchos proyectos y quiere generar capturas de pantalla de sus datos.
Actualmente genera un archivo PNG con 800x600 píxeles. Esto se puede ajustar usando los argumentos‘‘–width‘‘
y--height en la línea de órdenes. Se puede añadir un nombre de archivo después de --snapshot.
Opción de la línea de órdenes --lang
Basado en su configuración local, QGIS selecciona el idioma correcto. Si desea cambiar su idioma, se puede
especificar un código de idioma. Por ejemplo, --lang=it inica QGIS en idioma italiano.
Opción de la línea de órdenes --project
También es posible iniciar QGIS con un archivo de proyecto existente. Solamente agregue la opción --project
a la línea de comando, seguida por el nombre de su proyecto y QGIS se abrirá con todas las capas del archivo
indicado cargadas.
Opción de la línea de órdenes --extent
Use esta opción para iniciar con una extensión de mapa específica. Necesita añadir el cuadro delimitador de su
extensión en el siguiente orden, separado por una coma:
--extent xmin,ymin,xmax,ymax
Opción de la línea de órdenes --nologo
Este argumento de línea de órdenes oculta la pantalla de bienvenida cuando inicia QGIS.
Opción de la línea de órdenes --noplugins
Si tiene problemas con los complementos al iniciar, puede evitar cargarlos con ésta opción. Estarán aún disponibles
después en el administrador de complementos.
Opciónde la línea de órdenes --customizationfile
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Utilizando este argumento de línea de órdenes puede definir un archivo de personalizacion de la GUI, que se
utilizará al iniciar.
Opción de la línea de órdenes --nocustomization
Utilizando este argumento de línea de órdenes no se aplicará la personalización existente de la GUI.
Opción de la línea de órdenes --optionspath
Puede tener varias configuraciones y decidir cual utilizar al iniciar QGIS con esta opción. Véase Opciones para
confirmar donde almacena los archivos de configuración el sistema operativo. Actualmente, no hay forma de es-
pecificar un archivo para escribir la configuración; por lo tanto puede crear una copia del archivo de configuración
original y cambiarle el nombre. La opción especifica la ruta al directorio con los ajustes. Por ejemplo, para utilizar
el archivo de configuración /path/to/config/QGIS/QGIS2.ini , use la opción.
--optionspath /path/to/config/
Opción de la línea de órdenes --configpath
Esta opción es similar al anterior, pero además anula la ruta predeterminada para la configuración del usuario
(~/.qgis2) y fuerza QSettings para usar también este directorio. Esto permite a los usuarios, por ejemplo,
llevar la instalación de QGIS en una unidad flash junto con todos los complementos y configuraciones.
Opción de línea de comandos --código
Esta opción se puede utilizar para ejecutar un archivo python dado directamente después de que QGIS ha iniciado.
Por ejemplo, cuando se tiene un archivo python llamado load_alaska.py con el siguiente contenido:
from qgis.utils import iface
raster_file ="/home/gisadmin/Documents/qgis_sample_data/raster/landcover.img"
layer_name ="Alaska"
iface.addRasterLayer(raster_file, layer_name)
Suponiendo que esta en el directorio donde el archivo load_alaska.py se encuentra, puede iniciar QGIS,
cargue el archivo raster landcover.img y de a la capa el nombre ‘Alaska’ utilizando el siguiente comando:
qgis --code load_alaska.py
6.6 Proyectos
El estado de su sesión de QGIS es considerado un proyecto. QGIS trabaja en un proyecto cada vez. La con-
figuración está considerada por proyecto o como predeterminada para nuevos proyectos (ver sección Opciones).
QGIS puede guardar el estado de su espacio de trabajo dentro de un archivo de proyecto, usando las opciones de
menú Proyecto Guardar oProyecto Guardar como....
Cargar los proyectos guardados en una sesión de QGIS usando ProyectoAbrir...,Proyecto Nuevo a
partir de plantilla oProyecto Abrir reciente .
Si desea limpiar su sesión e iniciar una fresca, seleccione Proyecto Nuevo. Cualquiera de estas opciones le
pedirá que guarde el proyecto existente si se han hecho cambios desde que se abrió o se guardó por última vez.
El tipo de información guardada en el archivo de proyecto incluye:
Las capas añadidas
Que capas pueden ser consultadas
Propiedades de la capa, incluyendo simbolización y estilos
Proyección de la vista del mapa
Última extensión vista
Diseñador de impresión
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Elementos de diseñador de impresión con ajustes
Diseñador de impresión configuración de atlas
Configuración de digitalización
Tabla de relaciones
Proyectos Macros
Proyecto de estilos predeterminados
Configuración de complementos
Configuración de servidor QGIS desde la pestaña de ajustes de OWS en propiedades del proyecto
Consultas almacenadas en el Administrador de BBDD
El archivo del proyecto se guarda en formato XML, así es posible editarlo fuera de QGIS, si sabe lo que está
haciendo. El formato del archivo ha sido actualizado varias veces comparado con otras versiones de QGIS. Los
archivos de proyecto de versiones anteriores puede que ya no funcionen correctamente. Para estar al tanto de esto,
en la pestaña General bajo Configuración Opciones se puede seleccionar:
Preguntar si guardar cambios en el proyecto y la fuente de datos cuando sea necesario
Avisar al abrir un proyecto guardado con una versión anterior de QGIS
Siempre que guarde un proyecto en QGIS un respaldo del archivo del proyecto se hace con la extensión ~.
6.7 Salida
Hay muchas maneras de generar una salida desde su sesión QGIS. Ya hemos presentado una en la sección Proyec-
tos, guardando como un archivo de proyecto. Aquí hay una muestra de otras formas de producir archivos de
salida:
La opción del menú Proyecto Guardar como imagen abre un diálogo de archivo donde se selecciona el
nombre, la ruta y el tipo de imagen (PNG,JPG y muchos otros formatos). Un archivo de mundo con la
extensión PNGW o JPGW guardado en la misma carpeta la georreferencia de la imagen.
La opción del menú Proyecto Esportar a DXF ... abre un diálogo donde se puede definir el ‘Modo de
simbología’, la ‘Escala de la simbología’ y las capas vectoriales que se desea exportar a DXF. A través de los
símbolos ‘Modo de simbología’ desde la original simbología QGIS puede ser exportado con alta fidelidad.
La opción del menú Proyecto Nuevo diseñador de impresión abre un diálogo en donde se puede
diseñar e imprimir el lienzo de mapa actual (vea sección Diseñadores de impresión).
.
20 Chapter 6. Comenzar
CHAPTER 7
QGIS GUI
Cuando QGIS inicia, se le presenta la interfaz gráfica de usuario, como se muestra en la figura (los números del 1
al 5 en círculos amarillos se analiza más adelante).
Figure 7.1: QGIS GUI con datos de ejemplo de Alaska
Nota: Las decoraciones de las ventanas (barra de título, etc.) pueden ser distintas dependiendo de su sistema
operativo y su gestor de ventanas.
La GUI QGIS se divide en cinco zonas:
1. Barra de Menú
2. Barra de Herramientas
3. Leyenda del mapa
4. Vista del mapa
5. Barra de Estado
Estos cinco componentes de la interfaz de QGIS se describen con más detalle en la siguiente sección. Dos sec-
ciones más presentan atajos de teclado y ayuda contextual.
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7.1 Barra de Menú
La barra de menú permite el acceso a varias características de QGIS mediante un menú jerárquico estándar. Los
menús de nivel superior y un resumen de algunas opciones de menú se enumeran a continuación, junto con
los iconos asociados a medida que aparecen en la barra de herramientas, y los atajos de teclado. Los atajos
de teclado presentados en esta sección son los predeterminados; sin embargo, los atajos de teclado también se
pueden configurar manualmente utilizando el diálogo Configurar atajos del teclado, abrir desde Configuración
Configurar atajos de teclado....
Aunque la mayoría de las opciones tiene una herramienta correspondiente y viceversa, los menús no están orga-
nizados exactamente como las barras de herramientas. La barra de herramientas que contiene la herramienta esta
listada después de cada opción de menú como una entrada de casilla de verificación. Algunas opciones de menú
sólo aparecen si se carga el complemento correspondiente. Para obtener más información acerca de herramientas
y barra de herramientas, ver la sección Barra de herramietas.
7.1.1 Proyecto
Menú Opción Atajos Referencia Barra de herramietas
Nuevo Ctrl+N ver Proyectos Proyecto
Abrir Ctrl+O ver Proyectos Proyecto
Nuevo a partir de plantilla ver Proyectos Proyecto
Abrir recientes ver Proyectos
Guardar Ctrl+S ver Proyectos Proyecto
Guardar como... Ctrl+Shift+S ver Proyectos Proyecto
Guardar como imagen... ver Salida
Exportar DXF ... ver Salida
Nuevo diseñador de impresión Ctrl+P ver Diseñadores de impresión Proyecto
Administrador de diseñadores ... ver Diseñadores de impresión Proyecto
Diseñadores de impresión ver Diseñadores de impresión
Salir de QGIS Ctrl+Q
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7.1. Barra de Menú 23
QGIS User Guide, Publicación 2.8
7.1.2 Editar
Menú Opción Atajos Referencia Barra de
herramietas
Deshacer Ctrl+Z ver Advanced digitizing Digitalización
Avanzada
Rehacer Ctrl+Shift+Zver Advanced digitizing Digitalización
Avanzada
Cortar objetos espaciales Ctrl+X ver Digitizing an
existing layer
Digitalización
Copiar objetos espaciales Ctrl+C ver Digitizing an
existing layer
Digitalización
Pegar objetos espaciales Ctrl+V ver Digitizing an
existing layer
Digitalización
Pegar objetos espaciales como ver Working with the
Attribute Table
Añadir objetos espaciales Ctrl+. ver Digitizing an
existing layer
Digitalización
Mover objeto(s) espaciales ver Digitizing an
existing layer
Digitalización
Borrar seleccionados ver Digitizing an
existing layer
Digitalización
Girar objetos espacial(es) ver Advanced digitizing Digitalización
Avanzada
Simplificar objeto espacial ver Advanced digitizing Digitalización
Avanzada
Añadir anillo ver Advanced digitizing Digitalización
Avanzada
Añadir parte ver Advanced digitizing Digitalización
Avanzada
Rellenar anillo ver Advanced digitizing Digitalización
Avanzada
Borrar anillo ver Advanced digitizing Digitalización
Avanzada
Borrar parte ver Advanced digitizing Digitalización
Avanzada
Remodelar objetos espaciales ver Advanced digitizing Digitalización
Avanzada
Desplazar curva ver Advanced digitizing Digitalización
Avanzada
Dividir objetos espaciales ver Advanced digitizing Digitalización
Avanzada
Dividir partes ver Advanced digitizing Digitalización
Avanzada
Combinar objetos espaciales
seleccionados
ver Advanced digitizing Digitalización
Avanzada
Combinar los atributos de los objetos
espaciales seleccionados
ver Advanced digitizing Digitalización
Avanzada
Herramienta de nodos ver Digitizing an
existing layer
Digitalización
Rotar símbolos de putos ver Advanced digitizing Digitalización
Avanzada
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Después de activar el modo Conmutar edición de una capa, encontrará el icono Añadir objeto espacial
en el menú Edición dependiendo del tipo de capa (punto, línea o polígono).
7.1.3 Edición (extra)
Menú Opción Atajos Referencia Barra de herramietas
Añadir objetos espaciales ver Digitizing an existing layer Digitalización
Añadir objeto espacial ver Digitizing an existing layer Digitalización
Añadir objeto espacial ver Digitizing an existing layer Digitalización
7.1.4 Ver
Menú Opción Atajos Referencia Barra de
herramietas
Desplazar mapa Navegación de
mapas
Desplazar mapa a la
selección
Navegación de
mapas
Acercar zum Ctrl++ Navegación de
mapas
Alejar zum Ctrl+- Navegación de
mapas
Seleccionar ver Seleccionar y deseleccionar objetos
espaciales
Atributos
Identificar objetos
espaciales
Ctrl+Shift+I Atributos
Medir ver Mediciones Atributos
Zum General Ctrl+Shift+F Navegación de
mapas
Zum a la capa Navegación de
mapas
Zum a la selección Ctrl+J Navegación de
mapas
Zum anterior Navegación de
mapas
Zum siguiente Navegación de
mapas
Zum al tamaño real Navegación de
mapas
Ilustraciones ver Elementos decorativos
Modo Vista previa
Avisos del mapa Atributos
Nuevo marcador Ctrl+B ver Marcadores espaciales Atributos
Mostrar marcadores Ctrl+Shift+Bver Marcadores espaciales Atributos
Actualizar F5 Navegación de
mapas
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QGIS User Guide, Publicación 2.8
7.1.5 Capa
Menú Opción Atajos Referencia Barra de
herramietas
Crear capa ver Creating new Vector
layers
Administrar
Capas
Añadir capa Administrar
Capas
Empotrar capas y grupos ... ver Anidar proyectos
Añadir desde archivo de definición de
capa ...
Copiar estilo ver Style Menu
Pegar estilo ver Style Menu
Abrir Tabla de atributos ver Working with the
Attribute Table
Atributos
Conmutar edición ver Digitizing an existing
layer
Digitalización
Guardar cambios de la capa ver Digitizing an existing
layer
Digitalización
Ediciones actuales ver Digitizing an existing
layer
Digitalización
Guardar como...
Guardar como archivo de definición
de capa...
Eliminar Capa/Grupo Ctrl+D
Duplicar capa(s)
Establecer escala de visibilidad de
las capas
Establecer el SRC de la capa(s) Ctrl+Shift+C
Establecer SRC del proyecto a partir
de capa
Propiedades ...
Consulta...
Etiquetado
Añadir a la vista general Ctrl+Shift+O Administrar
Capas
Añadir todo a la vista general
Eliminar todo de la vista general
Mostrar todas las capas Ctrl+Shift+U Administrar
Capas
Ocultar todas las capas Ctrl+Shift+H Administrar
Capas
Mostrar capas seleccionadas
Ocultar capas seleccionadas
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7.1.6 Configuración
Menú Opción Atajos Referencia Barra de
herramietas
Paneles ver Paneles y Barras de
Herramientas
Barras de herramientasver Paneles y Barras de
Herramientas
Alternar el modo de pantalla
completa
F 11
Propiedades del proyecto... Ctrl+Shift+Pver Proyectos
SRC Personalizado ... ver Custom Coordinate Reference
System
Administrador de estilos... ver Presentation
Configurar atajos de teclado
...
Personalización ... ver Personalización
Opciones ... ver Opciones
Opciones de autoensamblado ...
7.1.7 Complementos
Menú Opción Atajos Referencia Barra de
herramietas
Administrar e Instalar
complementos ...
ver El diálogo de
complementos
Consola de Python Ctrl+Alt+P
Cuando inicie QGIS por primera vez no se cargan todos los complementos básicos.
7.1.8 Vectorial
Menú Opción Atajos Referencia Barra de herramietas
menuselection:Open Street Map –> ver Loading OpenStreetMap Vectors
Herramientas de análisis ver Complemento fTools
Herramientas de investigación ver Complemento fTools
Herramientas de Geoproceso ver Complemento fTools
Herramientas de geometría ver Complemento fTools
Herramientas de gestión de datos ver Complemento fTools
Cuando inicie QGIS por primera vez no se cargan todos los complementos básicos.
7.1.9 Ráster
Menú Opción Atajos Referencia Barra de herramietas
Calculadora ráster... ver Calculadora Ráster
Cuando inicie QGIS por primera vez no se cargan todos los complementos básicos.
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7.1.10 Base de datos
Menú Opción Atajos Referencia Barra de herramietas
Base de datosver Complemento administrador de BBDD Base de datos
Cuando inicie QGIS por primera vez no se cargan todos los complementos básicos.
7.1.11 Web
Menú Opción Atajos Referencia Barra de herramietas
Metabuscador ver metabuscador Web
Cuando inicie QGIS por primera vez no se cargan todos los complementos básicos.
7.1.12 Procesado
Menú Opción Atajos Referencia Barra de
herramietas
Caja de herramientas de
procesado
ver La caja de herramientas
Modelador gráfico ... ver &Modelador gráfico...
Historial y registro ... ver El administrador del historial
Opciones ... ver Configurar el entorno de trabajo de
procesamiento
Visor de resultador ... ver Configurar aplicaciones externas
Comandos Ctrl+Alt+Mver Los Comandos QGIS
Cuando inicie QGIS por primera vez no se cargan todos los complementos básicos.
7.1.13 Ayuda
Menú Opción Atajos Referencia Barra de herramietas
Contenido de la ayuda F1 Ayuda
¿Qué es esto? Shift+F1 Ayuda
Documentación de la API
¿Necesita soporte comercial?
Página web de QGIS Ctrl+H
Comprobar versión de QGIS
Acerca de
Patrocinadores de QGIS
Tenga en cuenta que para Linux , los elementos de la barra de menú mencionados anteriormente están de
manera predeterminada en la ventada de administrador KDE. En GNOME, el menú Configuración tiene diferente
contenido y los elementos que se encuentran aquí:
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SRC personalizado Editar
Administrador de estilos Editar
Configurar teclas de atajo Editar
Personalización Editar
Opciones Editar
Opciones de autoensamblado ... Editar
7.2 Barra de herramietas
La barra de herramientas proporciona acceso a la mayoría de las mismas funciones como las de los menús, y
herramientas adicionales para interactuar con el mapa. Cada elemento de la barra de herramientas tiene ayuda
emergente disponible. Mantenga el puntero del ratón sobre el elemento y una breve descripción del propósito de
la herramienta se mostrará.
Cada barra de menú se puede mover de acuerdo a sus necesidades. Además cada barra de menú se puede apagar
utilizando el menú contextual del botón derecho del ratón, sosteniendo el ratón sobre la barra de herramientas
(leer también Paneles y Barras de Herramientas).
Truco: Restauración de barras de herramientas
Si ha ocultado accidentalmente todas las barras de herramientas, puede recuperarlos eligiendo la op-
ción del menú Configuración Barra de herramientas . Si una barra de herramientas desaparece
bajo Windows, que parece ser un problema en QGIS de vez en cuando, tiene que quitar la clave
\HKEY_CURRENT_USER\Software\QGIS\qgis\UI\state en el registro. Cuando reinicie QGIS, la
clave se escribirá de nuevo con el estado por defecto y todas las barras de herramientas serán visibles de nuevo.
7.3 Leyenda del mapa
La zona de la leyenda del mapa registra todas las capas en el proyecto. La casilla de verificación de cada entrada
de leyenda se puede utilizar para mostrar u ocultar la capa. La barra de herramientas de leyenda en la leyenda
del mapa esta lista le permite Añadir grupo,Manejo de visibilidad de la capa de todas las capas o manejo
de combinación de capas predefinidas, Filtrar leyenda por contenido de mapa,Expandir todo oComprimir
todo yEliminar capa de grupo. El botón le permite añadir vistas Preestablecidos en la leyenda.
Esto significa que puede elegir por mostrar alguna capa con categorización específica y añadir esta vista a la
lista de Preestablecidos. Para añadir una vista preestablecida simplemente haga clic en , elija Añadir
preestablecido... desde el menú desplegable y de un nombre al preestablecido. Después verá una lista con todos
los preestablecidos que puede llamar pulsando el botón .
Todos los preestablecidos añadidos están presentes en el diseño de impresión con el fin de permitirle crear un
diseño de mapa en base a sus puntos de vista específicos (ver Propiedades principales).
Una capa se puede seleccionar y arrastrar hacia arriba o hacia abajo en la leyenda para cambiar el orden. El orden-
z significa que las capas enlistadas más cerca de la parte superior de la leyenda son dibujadas sobre las capas que
figuran más abajo en la leyenda.
Nota: Este funcionamiento puede ser anulado por el panel ‘Orden de la capa’
Las capas en la ventana de leyenda se pueden organizar en grupos. Hay dos formas de hacer esto:
1. Pulse el icono para añadir un nuevo grupo. Escriba un nombre para el grupo y pulse Enter. Ahora
haga clic en una capa existente y arrástrelo al grupo.
2. Seleccionar algunas capas, al hacer clic derecho en la ventana de la leyenda y elegir Grupo Seleccionado.
Las capas seleccionadas serán colocadas automáticamente en un nuevo grupo.
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Para llevar una capa fuera de un grupo, puede arrastrar hacia afuera , o haga clic derecho sobre él y elija Subir
elemento al nivel superior.
La casilla de verificación para un grupo mostrará u ocultará todas las capas en el grupo al hacer clic.
El contenido del menú contextual del botón derecho depende si el elemento de leyenda seleccionada es un ráster
o una capa vectorial. Para las capas vectoriales de GRASS , Botón de edición no está disponible. Vea la sección
Digitizing and editing a GRASS vector layer para obtener información sobre la edición de capas vectoriales de
GRASS.
El menú del boton derecho del raton para capas ráster
Zum a la capa
Mostrar en la vista general
Zum a la mejor escala (100%)
Eliminar
Duplicar
Establecer escala de visibilidad de la capa
Establecer SRC de la capa
Establecer SRC del proyecto a partir de capa
Estilos
Guardar como ...
Guardar como archivo de definición de capa ...
Propiedades
Cambiar nombre
Además, de acuerdo con la posición y la selección de la capa
Mover al nivel superior
Grupo seleccionado
Menú del botón derecho del ratón para las capas vectoriales
Zum a la capa
Mostrar en la vista general
Eliminar
Duplicar
Establecer escala de visibilidad de la capa
Establecer SRC de la capa
Establecer SRC del proyecto a partir de capa
Estilos
Abrir tabla de atributos
Conmutar edición (no disponible para capas GRASS)
Guardar como ...
Guardar como Estilo de definición de capa
Filtrar
Mostrar el conteo de objetos espaciales
Propiedades
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Cambiar nombre
Además, de acuerdo con la posición y la selección de la capa
Mover al nivel superior
Grupo seleccionado
Menú del botón derecho del ratón para grupo de capas
Zum al grupo
Eliminar
Establecer SRC del grupo
Cambiar nombre
Añadir grupo
Es posible seleccionar mas de una capa o grupo al mismo tiempo manteniendo presionada la tecla Ctrl mientras
selecciona las capas con el botón izquierdo del ratón. Después puede mover todas las capas a un nuevo grupo al
mismo tiempo.
También puede eliminar más de una capa o un grupo a la vez seleccionando varias capas con la tecla Ctrl y
presionando Ctrl+D después. De esta manera, todas las capas o grupos seleccionados será eliminado de la lista
de capas.
7.3.1 Trabajar con el orden de la leyenda de la capa independiente
Hay un panel que le permite definir un orden dibujo independiente para la leyenda del mapa. Puede activarlo en el
menú ConfiguraciónPaneles Orden de Capas. Esta característica le permite, por ejemplo, ordenar sus capas
en orden de importancia, pero aún mostrarlas en el orden correcto (ver figure_layer_order) . Comprobación de la
caja Orden de control del renderizado debajo de la lista de capas causará una reversión en el comportamiento
predeterminado .
Figure 7.2: Definir el orden de la leyenda de una capa independiente
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7.4 Vista del mapa
Este es el “final del negocio” de QGIS — ¡los mapas son desplegados en esta zona! El mapa que se muestra
en esta ventana dependerá de las capas vectoriales y ráster que ha elegido cargar (ver secciones siguientes para
obtener más información sobre cómo cargar capas). La vista del mapa se puede desplazar, cambiar el enfoque de
la pantalla del mapa a otra región, y que se puede hacer zum dentro y fuera. Varias otras operaciones se pueden
realizar en el mapa como esta descrito en la descripción de la barra de herramientas anteriormente. La vista del
mapa y la leyenda están estrechamente vinculados entre sí — los mapas en vista reflejan los cambios que realice
en el área de leyenda .
Truco: Zum al mapa con la rueda del ratón
Puede utilizar la rueda del ratón para acercar y alejar zum en el mapa. Coloque el cursor del ratón dentro del mapa
y gire la rueda hacia adelante (hacia la derecha) para acercar y hacia atrás (hacia usted) para alejarlo. El zum se
centra en la posición del cursor del ratón. Puede personalizar el comportamiento del zum de la rueda del ratón
usando la pestaña Herramientas del mapa bajo el menú ConfiguraciónOpciones
Truco: Desplazar el mapa con las teclas de dirección y barra de espaciadora
Puede utilizar las teclas de flechas para desplazar el mapa. Coloque el cursor dentro del mapa y haga clic en la
tecla de flecha a la derecha para desplazarse al este, tecla de flecha izquierda para el oeste, flecha arriba para el
norte y flecha abajo al sur. Puede también desplazar el mapa utilizando la barra espaciadora o al hacer clic en la
rueda del ratón: basta con mover el ratón mientras mantiene pulsada la barra espaciadora o haga clic en la rueda
del ratón.
7.5 Barra de Estado
La barra de estado muestra la posición actual en coordenadas de mapa (por ejemplo, metros o grados decimales)
como el puntero del ratón se mueve a través de la vista del mapa. A la izquierda de la pantalla de coordenadas en
la barra de estado es un botón pequeño que alterna entre mostrar la posición en coordenadas y la extensión del la
vista del mapa como como desplazar, acercar y alejar zum.
Junto a la visualización de coordenadas se encuentra la visualización de la escala. Este muestra la escala de la
vista del mapa. Si acercar o alejar zum, QGIS muestra la escala actual. Hay un selector de escala, lo que le permite
elegir entre las escalas predefinidas de 1:500 a 1:1000000.
A la derecha de la escala desplegada se puede definir una rotación horaria actual de su vista de mapa en grados.
Una barra de progreso en la barra de estado muestra el progreso de representación, ya que cada capa se dibuja a la
vista del mapa. En algunos casos, como en la recopilación de estadísticas en capas ráster, la barra de progreso se
utiliza para mostrar el estado de las operaciones largas.
S un nuevo complemento o una actualizacion de complemento disponible, verá un mensaje en el extremo izquierdo
de la barra de estado. En el lado derecho de la barra de estado, hay una pequeña casilla de verificación que se
puede utilizar para evitar temporalmente capas siendo represtados a la vista del mapa (ver sección Renderizado
abajo). El icono detiene inmediatamente el proceso de representación del mapa actual.
A la derecha de las funciones de representación, vera el código EPSG de la actual proyección SRC y un icono de
proyector. Haga clic en este para abrir las propiedades de proyección del actual proyecto.
Truco: Calcular la escala correcta de su lienzo de mapa
Cuando inicia QGIS, las unidades son grados, y esto significa que QGIS interpretará cualquier coordenada en su
capa como se especifica en grados. Para obtener valores de escala correctos, se puede cambiar esta configuración a
metros manualmente en la pestaña General bajo ConfiguraciónPropiedades del Proyecto, o puede seleccionar
un proyecto SRC al hacer clic en el icono SRC Actual: en la esquina inferior derecha de la barra de estado. En el
último caso, las unidades se establecen en lo que esta especificado en la proyección del proyecto (e.g., ‘+units=m’).
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.
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34 Chapter 7. QGIS GUI
CHAPTER 8
Herramientas generales
8.1 Teclas de acceso rápido
QGIS proporciona atajos de teclado predeterminados para muchas características. Puede encontrarlos en la sec-
ción Barra de Menú. Además, la opción de menú Configuración Configurar atajos de teclado... permite
cambiar los atajos de teclado predeterminados y agregar otros nuevos a las características de QGIS .
Figure 8.1: Definir opciones de atajos (Gnome)
La configuración es muy simple. Solo seleccione una entidad de la lista y haga clic en [Cambiar],[Establecer
a ninguno] o[Establecer predeterminado]. Una vez finalizada la configuración, se puede guardar como un
archivo XML y cargarlo en otra instalación de QGIS.
8.2 Ayuda de contexto
Cuando necesite ayuda sobre un tema especifico, puede acceder a la ayuda de contexto mediante el botón [Ayuda]
disponible en la mayoría de diálogos – tenga en cuenta que los complementos de terceros pueden apuntar a paginas
web dedicadas.
8.3 Renderizado
Por omisión, QGIS representa todas las capas visibles siempre que se actualiza la vista del mapa. Los eventos que
desencadena una actualización de la vista del mapa incluyen:
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Añadir una capa
Desplazar o hacer zoom
Redimensionar la ventana de QGIS
Cambiar la visibilidad de una o varias capas
QGIS permite controlar el proceso de renderizado de diversas formas.
8.3.1 Renderizado dependiente de la escala
El renderizado dependiente de la escala le permite especificar las escalas mínima y máxima a las que una capa
será visible. Para establecer el renderizado dependiente de la escala, abra el diálogo Propiedades mediante doble
clic en una capa en el panel Capas. En la pestaña General, haga clic en la casilla Visibilidad dependiente de la
escala para activar la característica, luego establezca los valores mínimo y máximo de escala.
Puede determinar los valores de escala haciendo zum primero al nivel que quiera usar y anotanto el valor de escala
en la barra de estado de QGIS.
8.3.2 Controlar el renderizado del mapa
El renderizado del mapa se puede controlar de varias formas, como se describe a continuación.
Suspender el renderizado
Para suspender el renderizado, haga clic en la casilla Representar en la esquina inferior derecha de la barra de
estado. Cuando la casilla Representar no está marcada, QGIS no redibuja el lienzo en respuesta a cualquiera
de los eventos descritos en la sección Renderizado. Ejemplos de cuándo puede querer suspender la representación
incluyen:
Añadir muchas capas y simbolizarlas antes de dibujar
Añadir una o más capas grandes y establecer la dependencia de escala antes de dibujar
Añadir una o más capas grandes y hacer zoom a una vista específica antes de dibujar
Cualquier combinación de la anteriores
Marcar la casilla Renderizar habilita el renderizado y origina un refresco inmediato del lienzo del mapa.
Configurar la opción de añadir una capa
Puede establecer una opción para cargar siempre las nuevas capas sin dibujarlas. Esto significa que las capas se
añadirán al mapa pero su casilla de visibilidad en el panel Capas no estará marcada de forma predeterminada.
Para establecer esta opción, seleccione la opción de menú Configuración Opciones y haga clic en la pestaña
Representación. Desmarque la casilla Por omisión, las nuevas capas añadidas al mapa se deben visualizar.
Cualquier capa añadida posteriormente al mapa estará desactivada (invisible) por omisión.
Detener el renderizado
Para detener el dibujado del mapa, presione la tecla ESC. Esto detendrá el refresco del lienzo del mapa y dejará el
mapa parcialmente dibujado. Puede que tarde un poco desde que se presiona la tecla ESC hasta que se detenga el
dibujado del mapa.
Nota: Actualmente no es posible detener la representación — esto se desactivó en el paso a Qt4 debido a
problemas y cuelgues de la Interfaz de Usuario (IU).
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Actualizar la visualización del mapa durante el renderizado
Se puede establecer una opción para actualizar la visualización del mapa a medida que se dibujan los objetos
espaciales. Por omisión, QGIS no muestra ningún objeto espacial de una capa hasta que toda la capa ha sido
representada. Para actualizar la pantalla a medida que se leen los objetos espaciales desde el almacén de datos,
seleccione la opción de menú Configuración Opciones y haga clic en la pestaña Representación. Establezca
el número de objetos espaciales a un valor apropiado para actualizar la pantalla durante la representación. Al
establecer un valor de 0 desactiva la actualización durante el dibujado (este es el valor predeterminado). Es-
tablecer un valor demasiado bajo dará como resultado un bajo rendimiento, ya que la vista del mapa se actualiza
continuamente durante la lectura de los objetos espaciales. Un valor sugerido para empezar es 500.
Influir en la calidad del renderizado
Para influir en la calidad de la presentación del mapa, se tienen dos opciones. Elegir la opción de menú Configu-
ración Opciones, hacer clic en la pestaña Representación y seleccionar o deseleccionar las siguientes casillas
de verificación:
Hacer que las líneas se muestren menos quebradas a expensas del rendimiento de la representación
Solucionar problemas con polígonos rellenados incorrectamente
Acelerar renderizado
Hay dos ajustes que le permiten mejorar la velocidad de presentación. Abrir el diálogo de las opciones de QGIS
usando ConfiguraciónOpciones, ir a la pestaña guilabel:Representación y seleccionar o deseleccionar las sigu-
ientes casillas de verificación:
Activar buffer trasero. Esto proporciona un mejor rendimiento gráficos a costa de perder la posibilidad
de cancelar la representación y dibujar objetos espaciales incrementalmente. Si no esta marcada, se puede
establecer el Número de objetos espaciales a dibujar antes de actualizar la visualización, de lo contrario
esta opción está inactiva.
Usar cacheado de representación cuando sea posible para acelerar redibujados
8.4 Mediciones
Las mediciones funcionan en sistemas de coordenadas proyectadas (por ejemplo, UTM) y en datos sin proyectar.
Si el mapa cargado está definido con un sistema de coordenadas geográficas (latitud/longitud), los resultados
de las mediciones de lineas o áreas serán incorrectos. Para solucionar esto, se debe establecer un sistema de
coordenadas del mapa apropiado (ver sección :ref:label_projections). Todos los módulos de medición también
usan la configuración de autoensamblado del módulo de digitalización. Esto es útil si se quiere medir a lo largo
de lineas o áreas en una capa vectorial.
Para seleccionar una herramienta de medición, pulsar y seleccione la herramienta que se quiera usar.
8.4.1 Medir longitud, áreas y ángulos
Medir Linea: En QGIS es posible medir distancias reales entre puntos dados conforme a un elipsoide definido.
Para configurar esto, seleccione la opción del menú Configuración Opciones, haga clic en la pestaña Her-
ramientas del mapa y seleccione el elipsoide apropiado. Ahí, también puede definir un color de banda de medida
y las unidades de medida preferida (metros o pies) y las unidades de ángulos (grados, radianes, grados centesi-
males). La herramienta entonces le permite hacer puntos en el mapa. La longitud de cada segmento, así como el
total, aparecerá en la ventana de medición. Para detener la medición, haga clic en el botón derecho del ratón. Tenga
en cuenta que puede cambiar interactivamente las unidades de medición en el diálogo. Se anula las Unidades de
8.4. Mediciones 37
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medición preferida en las opciones. Hay una sección de información en el diálogo que muestra que configuración
de SRC se esta utilizando durante los cálculos de medición.
Figure 8.2: Medir distancia (Gnome)
Medir áreas: Las áreas también pueden ser medidas. En la ventana de medición, aparece el tamaño del área
acumulada. Además, la herramienta de medición se autoensamblará a la capa actualmente seleccionada, siempre
que la capa tenga establecida una tolerancia de autoensamblado (ver sección Setting the Snapping Tolerance and
Search Radius). Por lo tanto, si se desea medir con exactitud a lo largo de un objeto espacial lineal, o alrededor
de un objeto poligonal, primero establezca su tolerancia de autoensamblado, luego seleccione la capa. Ahora, al
utilizar las herramientas de medición, cada clic del ratón (dentro de la tolerancia configurada) se ajustará a esa
capa.
Figure 8.3: Medir área (Gnome)
Medir ángulo: Se pueden también medir ángulos. El cursor se convierte en forma de cruz. Se debe hacer clic para
dibujar el primer segmento del ángulo que se desea medir y a continuación mover el cursor para dibujar el ángulo
deseado. La medida se mostrará en el diálogo emergente.
Figure 8.4: Medir ángulo (Gnome)
8.4.2 Seleccionar y deseleccionar objetos espaciales
La barra de herramientas de QGIS provee varias herramientas para seleccionar objetos espaciales en la vista del
mapa. Para seleccionar una o varios objetos, basta con hacer clic en y seleccionar la herramienta:
Seleccionar objetos espaciales individuales
Seleccionar objetos espaciales por rectángulo
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Seleccionar objetos espaciales por polígono
Seleccionar objetos espaciales a mano alzada
Seleccionar objetos espaciales por radio
Para deseleccionar todos los objetos espaciales seleccionados, haga clic en
Deseleccionar objetos espaciales de todas las capas.
Seleccionar un objeto espacial utilizando una expresión‘permite al usuario seleccionar objetos espaciales utilizando un dialogo de expresión. Ver capítulo :ref:‘vector_expressions
para más ejemplos.
Los usuarios pueden guardar objetos espaciales seleccionados en una Nueva capa vectorial en memoria o una
Nueva capa vectorial utilizando Edición Pegar objetos espaciales como ... y elegir el modo que desea.
8.5 Identificar objetos espaciales
La herramienta de identificar le permite interactuar con la vista del mapa y obtener información de los objetos es-
paciales en una ventana emergente. Para identificar objetos espaciales, se usa Ver Identificar objetos espaciales
o presionar Ctrl + Shift + I, o hacer clic en el icono Identificar objetos espaciales en la barra de herramientas.
Si hace clic en varios objetos, el diálogo Resultados de la identificación se enumerará la información acerca de
todos los objetos seleccionados. El primer elemento es el numero de la capa en la lista de resultados, seguido por
el nombre. Luego su primer hijo será el nombre de un campo con su valor. El primer campo es la seleccionada en
PropiedadesVisualizar. Al final, toda la información sobre el objeto se mostrará.
Esta ventana puede ser personalizada para mostrar campos personalizados, pero por omisión mostrará tres tipos
de información:
Acciones: se pueden agregar acciones a la ventana para identificar objetos espaciales. Al hacer clic en la
etiqueta de la acción, ésta se llevará a cabo. Por omisión, sólo se añade una acción, para ver el formulario
del objeto para edición.
Derivado: esta información se calcula o es derivada de otra información. Se puede encontrar las coordenadas
pulsadas, coordendas X y Y, área y perímetro en unidades del mapa para polígonos, longitud en unidades
del mapa para lineas e ID de los objetos espaciales.
Atributos de datos: Esta es la lista de campos de atributos de los datos.
Figure 8.5: Diálogo de identificación de objetos espaciales (Gnome)
En la parte superior de la ventana, tiene cinco iconos:
Expandir árbol
8.5. Identificar objetos espaciales 39
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Comprimir árbol
Comportamiento predeterminado
Copiar atributos
Imprimir respuesta del HTML seleccionado
En la parte inferior de la ventana, se tiene las listas desplegables de Modo yVista. Con la lista desplegable Modo
se puede definir el modo de la identificación: ‘Capa actual’, ‘De arriba a abajo, irse en el primero’, ‘De arriba a
abajo’ y ‘Selección de capa’. La Vista se puede establecer como ‘Árbol’, ‘Tabla’ y ‘Gráfico’.
La herramienta de identificación le permite abrir automáticamente un formulario. En este modo se puede cambiar
los atributos de los objetos espaciales.
Otras funciones se pueden encontrar en el menú contextual del elemento identificado. Por ejemplo, del menú
contextual se puede:
Ver el formulario del objeto espacial
Zum a objeto espacial
Copiar objeto espacial: Copiar toda la geometría y atributos del objeto espacial
Selección de objetos espaciales: añadir objeto espacial identificado a selección.
Copiar el valor del atributo: copiar solo el valor del atributo sobre el cual se hizo clic
Copiar atributos del objeto espacial: Copiar solo atributos
Limpiar resultados: quitar resultados de la ventana
Limpiar resaltados: Deseleccionar los objetos espaciales en el mapa
Resaltar todo
Resaltar capa
Activar capa: Elegir una capa para ser activada
Propiedades de la capa: Abrir la ventana de propiedades de la capa.
Expandir todo
Colapsar todo
8.6 Elementos decorativos
Las Ilustraciones de QGIS incluyen la Cuadrícula, Etiqueta de Copyright, Flecha de Norte y Barra de Escala. Se
usan para ‘adornar’ el mapa al agregar elementos cartográficos.
8.6.1 Cuadrícula
Cuadrícula permite agregar una rejilla de coordenadas y anotaciones a la vista del mapa.
1. Seleccione en el menú Ver IlustracionesCuadrícula. Aparece el díálogo (ver figure_decorations_1).
2. Activar la casilla Activar cuadrícula y establecer la definición de la cuadrícula de acuerdo con las capas
cargadas en la vista del mapa.
3. Activar la casilla Dibujar anotaciones y establecer la definición de las anotaciones de acuerdo con las
capas cargadas en la vista del mapa.
4. Hacer clic en [Aplicar] para verificar que se vea como se esperaba.
40 Chapter 8. Herramientas generales
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Figure 8.6: El diálogo de cuadrícula
5. Pulse [Aceptar] para cerrar el diálogo.
8.6.2 Etiqueta de derechos de autor
Etiqueta de copyright añade una etiqueta de copyright usando el texto que se prefiera al mapa.
Figure 8.7: Diálogo de copyright
1. Seleccione en el menú Ver IlustracionesEtiqueta de Copyright. Aparece el díálogo (ver fig-
ure_decorations_2).
2. Escribir el texto que se quiera colocar en el mapa. Se puede usar HTML como se muestra en el ejemplo.
3. Elegir la ubicación de la etiqueta en la lista desplegable Ubicación
4. Comprobar que la casilla de verificación Activar etiqueta de copyright este marcada.
5. Hacer clic en [Aceptar]
En el ejemplo anterior, que es el predeterminado, QGIS coloca un símbolo de los derechos de autor seguido de la
fecha en la esquina inferior derecha de la vista del mapa.
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8.6.3 Flecha del Norte
Flecha de Norte coloca una sencilla flecha de norte en la vista del mapa. En la actualidad sólo hay un estilo
disponible. Se puede ajustar el ángulo de la flecha o dejar que QGIS establezca la dirección automáticamente.
Si decide dejar que QGIS determine la dirección, hará su mejor conjetura en cuanto a cómo se debe orientar la
flecha. Para la colocación de la flecha, se tienen cuatro opciones que corresponden a las cuatro esquinas de la vista
del mapa.
Figure 8.8: Diálogo de la flecha del Norte
8.6.4 Barra de escala
Barra de escala añade una barra de escala sencilla a la vista del mapa. Se puede controlar el estilo y la ubicación,
así como el etiquetado de la barra.
Figure 8.9: El diálogo de barra de escala
QGIS sólo es compatible con la visualización de la escala en las mismas unidades que el marco del mapa. Así que
si las unidades de las capas están en metros, no se puede crear una barra de escala en pies. Del mismo modo, si
está usando grados decimales, no se puede crear una barra de escala para mostrar la distancia en metros.
Para añadir una barra de escala:
1. Seleccionar del menú Ver Ilustraciones Barra de escala. Se iniciará el diálogo (ver fig-
ure_decorations_4).
2. Elegir la ubicación de la lista desplegable Ubicación .
3. Elegir el estilo de la caja desplegable Estilo de la barra de escala
4. Seleccionar el color de la barra Color de la barra o usar el color negro predetermi-
nado.
42 Chapter 8. Herramientas generales
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5. Establecer el tamaño de la barra y su etiqueta Tamaño de barra .
6. Comprobar que la casilla de verificación Habilitar barra de escala esté marcada.
7. Opcionalmente, comprobar Redondear números automáticamente al cambiar de tamaño.
8. Hacer clic en [Aceptar]
Truco: Configuración de elementos decorativos
Al guardar un proyecto .qgs, cualquiera de los cambios que se hayan hecho a la cuadrícula, flecha de norte, barra
de escala y copyright se guardarán en el proyecto y se restaurán la próxima vez que cargue el proyecto.
8.7 Herramientas de anotaciones
La herramienta Anotación de texto en la barra de herramientas de atributos provee la posibilidad de colocar texto
con formato en un globo en la vista del mapa de QGIS. Usando la herramienta Anotación de texto haga clic en la
vista del mapa.
Figure 8.10: Diálogo de texto de anotación
Haciendo doble clic sobre el elemento se abre un cuadro de diálogo con varias opciones. Hay un editor de texto
para escribir el texto con formato y otros ajustes de elementos. Por ejemplo, existe la opción de tener el elemento
colocado en una posición del mapa (mostrado por el símbolo del marcador) o tener el elemento en una posición
de la pantalla (no relacionado con el mapa). El elemento se puede mover por la posición del mapa (al arrastrar el
marcador del mapa) o moviendo solo el globo. Los iconos son parte del tema de los SIG, y se utilizan de forma
predeterminada en otros temas también.
La herramienta Mover anotación permite mover la anotación en la vista del mapa.
8.7.1 Anotaciones HTML
La herramienta Anotación HTML de la barra de herramientas de atributos provee la posibilidad de colocar el
contenido de un archivo HTML en un globo en la vista del mapa de QGIS. Utilizando la herramienta Anotación
HTML, haga clic en la vista del mapa y agregue la ruta de acceso al archivo HTML en el diálogo.
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8.7.2 Anotaciones SVG
La herramienta Anotación SVG de la barra de herramientas de atributos provee la posibilidad para colocar un
símbolo SVG en un globo en la vista del mapa de QGIS. Utilizando la herramienta Anotación SVG, haga clic en
la vista del mapa y añada la ruta de acceso al archivo SVG en el diálogo.
8.7.3 Anotaciones de formulario
Además, puede crear sus propios formularios de anotaciones. La herramienta Formulario de anotaciones
es util para mostrar los atributos de una capa vectorial en un formulario Qt Designer personalizado
(ver figure_custom_annotation). Esto es similar al diseñador de formularios para la herramienta Iden-
tificar objetos espaciales, pero mostrado en un elemento de la anotación. Ver también el video
https://www.youtube.com/watch?v=0pDBuSbQ02o de Tim Sutton para más información.
Figure 8.11: Formulario de anotación de diseñador qt personalizado
Nota: Si presiona Ctrl+T mientras está activa una herramienta Anotación (mover anotación, anotación de texto,
anotación de formulario), se invierten los estados de visibilidad de los elementos.
8.8 Marcadores espaciales
Los marcadores espaciales le permiten “marcar” una localización geográfica y volver a ella más tarde.
8.8.1 Crear un marcador
Para crear un marcador:
1. Hacer zoom o desplazarse al área de interés.
2. Seleccione la opción de menú Ver Nuevo marcador o presione Ctrl-B.
44 Chapter 8. Herramientas generales
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3. Introduzca un nombre descriptivo para el marcador (hasta 255 caracteres).
4. Presione Añadir para añadir el marcador o [Borrar] para eliminarlo.
Tenga en cuenta que puede tener múltiples marcadores con el mismo nombre.
8.8.2 Trabajar con marcadores
Para usar o administrar marcadores, seleccionar la opción de menú Ver Mostrar marcadores. El cuadro de
diálogo Marcadores geoespaciales permite hacer zum a un marcador o eliminarlo. No se pueden editar el nombre
o las coordenadas del marcador.
8.8.3 Hacer zoom a un marcador
En el diálogo Marcadores geoespaciales, seleccione el marcador deseado haciendo clic en él y luego en [Zum a].
También puede hacer zum a un marcador haciendo doble clic en él.
8.8.4 Borrar un marcador
Para eliminar un marcador del cuadro de diálogo Marcadores geospaciales, hacer clic sobre él, después hacer clic
en [Eliminar]. Confirmar la elección pulsando [Si], o cancelar la eliminación pulsando [No].
8.8.5 Importar o exportar un marcador
Para compartir o transferir sus marcadores entre computadoras que puede utilizar el menú desplegable Compartir
en el diálogo Marcadores Geoespaciales.
8.9 Anidar proyectos
Si se quiere incluir contenido de otros proyectos en un proyecto, se puede elegir Capa Empotrar capas y
grupos.
8.9.1 Empotrar capas
El siguiente cuadro de diálogo le permite incluir capas de otros proyectos. Aquí un pequeño ejemplo:
1. Presione para buscar otro proyecto del conjunto de datos de Alaska.
2. Seleccionar el archivo de proyecto grassland. Puede ver el contenido del proyecto (ver fig-
ure_embed_dialog).
3. Presionar Ctrl y hacer clic sobre las capas file:grassland yregions. Presionar [OK]. Ahora la capa
seleccionada está incrustada en la leyenda del mapa y la vista del mapa.
Si bien las capas incrustadas son editables, no se pueden cambiar sus propiedades como estilo y etiquetado.
8.9.2 Eliminar capas incrustadas
Clic derecho en la capa empotrada y elegir Eliminar.
.
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Figure 8.12: Seleccionar capas y grupos para empotrar
46 Chapter 8. Herramientas generales
CHAPTER 9
Configuración QGIS
QGIS es altamente configurable a través del menú Configuración. Elegir entre Paneles, Barras de herramientas,
Propiedades del proyecto, Opciones y Personalización.
Nota: QGIS sigue las directrices de escritorio de localización de opciones y elementos de las propiedades del
proyecto. Por consecuencia relacionada a el sistema operativo que utiliza, la ubicación de algunos elementos
descritos anteriormente podrían estar situados en el menú Ver (Paneles y Barra de herramientas) o en Opciones
Proyecto.
9.1 Paneles y Barras de Herramientas
En el menú Paneles, puede encender o apagar los widgets de QGIS. El menú Barra de herramientas
proporciona la posibilidad para encender y apagar grupos de iconos en la barra de herramientas (ver fig-
ure_panels_toolbars).
Figure 9.1: El menú de paneles y barras de herramientas
47
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Truco: Activar la información general de QGIS
En QGIS, puede usar un panel de vista general que proporciona una vista completa de las capas añadidas. Se
puede seleccionar en el menú Configuración Paneles oVer Paneles. Dentro de la vista un rectangulo
mostrará la vista del mapa actual. Esto le permite determinar rápidamente que área del mapa se ve actualmente.
Tenga en cuenta que las etiquetas no son representadas en la vista general del mapa incluso si las capas en la vista
general del mapa se ha establecido el etiquetado. Al hacer clic y arrastrar el rectángulo rojo en la vista general se
muestra la extensión actual, la vista principal del mapa se actualizará en consecuencia.
Truco: Mostrar el registro de mensajes
Es posible seguir los mensajes de QGIS. Puede activar Registro de mensajes en el menú Configuración
Paneles oVer Paneles y seguir los mensajes que aparecen en las diferentes pestañas durante la carga y
funcionamiento.
9.2 Propiedades del proyecto
En la ventana de propiedades del proyecto bajo Configuración Propiedades del proyecto (kde) o
Proyecto Propiedades del proyecto (Gnome), puede establecer opciones específicas del proyecto. Estos in-
cluyen:
En le menú General, el título del proyecto, color de selección y fondo, unidades de la capa, la precisión y
la opción de guardar rutas relativas a las capas se pueden definir. Si la trasformación SRC esta activada,
se puede elegir un elipsoide para cálculos de distancia. Se pueden definir las unidades del lienzo(sólo se
utiliza cuando la transformación SRC está desactivada) y la precisión de decimales se utiliza. También
puede definir una lista de la escala del proyecto, que anula las escalas predefinidas globales.
El menú SRC habilitado para elegir el Sistema de Referencia de Coordenadas para este proyecto, y para
habilitar la reproyección al vuelo de capas ráster y vector cuando se muestran capas de un diferente SRC.
Con el tercer menú Identificar capas, se establece (o deshabilitar) las capas que responderán a la herramienta
de identificar objetos espaciales (ver el párrafo de “Herramientas del mapa” de la sección Opciones para
permitir la identificación de múltiples capas)
El menú Estilos predeterminados le permite controlar cómo las nuevas capas se dibujaran cuando no tienen
un estilo existente .qml definido. También puede establecer el nivel de trasparecía por defecto para nuevas
capas y si los símbolos deben tener colores al azar para asignarlos. También hay una sección adicional
donde puede definir colores específicos para el proyecto en ejecución. Puede encontrar los colores añadidos
en el menú desplegable de la ventana de diálogo de color presente en cada representación.
La pestaña de Servidor OWS le permite definir información acerca del QGIS servidor WMS y capacidades
WFS, extensión y restricciones SRC.
El menú Macros es utilizado para editar macros de Python para proyectos. Actualmente, solo tres macros
están disponibles: openProject(),saveProject() and closeProject().
El menú Relaciones es utilizado para definir relaciones 1:n. Las relaciones están definidas en el diálogo de
propiedades del proyecto. Una vez que existen las relaciones de una capa, un nuevo elemento de la interfaz
de usuario en la vista del formulario (por ejemplo al identificar un elemento espacial y abrir el formulario)
mostrará una lista de las entidades relacionadas. Este proporciona un poderosa forma para expresar, por
ejemplo la inspección de la longitud de una tubería o el segmento de carretera. Se puede encontrar más
información acerca de relaciones 1:n y soporte en la sección Creating one to many relations.
48 Chapter 9. Configuración QGIS
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Figure 9.2: Ajustes de la Macro en QGIS
9.3 Opciones
Algunas opciones básicas de QGIS se pueden seleccionar utilizando el diálogo Opciones. Seleccione la opción
del menú Configuración Opciones. Las pestañas donde puede personalizar las opciones están descritas a
continuación.
9.3.1 Menú General
Aplicación
Seleccione el Estilo (QGIS requiere reiniciar) y elija entre ‘Oxygen’,’Windows’,’Motif’,’CDE’,
‘Plastique’ and ‘Cleanlooks’ ( ).
Definir el Tema de icono . Actualmente solo ‘predeterminado’ es posible.
Definir el Tamaño del icono .
Definir la Fuente. Elegir entre Qt default y una fuente definida por el usuario.
Cambiar el Límite de tiempo para mensajes o diálogos con tiempo .
Ocultar la pantalla de bienvenida al iniciar la aplicación
Mostrar consejos al iniciar
Títulos de cajas de grupos en negrita
Cajas de grupo al estilo QGIS
Utilizar los diálogos de selección de colores nativos
Usar diálogos de selección de color actualizadas en vivo
Estilo de barra lateral personalizado
Apoyo para rotación del lienzo experimental (requiere reiniciar)
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Los archivos de proyecto
Abrir proyecto on launch (elegir entre ‘Nuevo’, Más reciente’ y ‘Específico’). Al elegir ‘Específico’
utilice el para definir un proyecto.
Crear nuevo proyecto desde el proyecto predeterminado. Tiene la posibilidad de presionar Establecer
el actual proyecto como predeterminado o sobre Restablecer el predeterminado. Puede navegar a través de
sus archivos y definir un directorio donde se encuentra las plantillas definidas por el usuario. Esto se añadirá
aProyecto Nueva plantilla de formulario. Si activa primero Crear nuevo proyecto desde proyecto
predeterminado y entonces guarde un proyecto en l la carpeta de las plantillas de proyecto.
Solicitar guardar proyectos y fuentes de datos modificadas cuando sea necesario
Pedir confirmación cuando se va a eliminar una capa
Avisar cuando se abra un proyecto guardado con una versión anterior de QGIS
Habilitar macros . Esta opción fue creada para manejar macros que estén escritos para llevar una
acción en los eventos del proyecto. Puede elegir entre ‘Nunca’, ‘Preguntar’, ‘Sólo para esta sesión’ y
‘Siempre (no recomendado)’.
9.3.2 Menú Sistema
Entorno
Variables de entorno del sistema ahora se puede ver, y muchos lo configuran en el grupo Entorno (ver fig-
ure_environment_variables). Esto es útil para las plataformas, como Mac, donde una aplicación GUI no heredan
necesariamente entorno del casco del usuario. También es útil para configurar y visualizar las variables de en-
torno para los conjuntos de herramientas externas controladas por la caja de herramientas de procesamiento (por
ejemplo, SAGA, GRASS), y para activar la salida de depuración para secciones específicas del código fuente.
Utilizar variables personalizadas (requiere reiniciar - incluir separadores). Puede [Añadir] y[Borrar]
variables. Las variables de entorno ya definidas se muestran en Variables de entorno actuales, y es posible
filtrarlos activando Mostrar sólo variables de QGIS específicas.
Rutas de complemento
[Añadir] o[Borrar] Ruta(s) para buscar librerías de componentes en C++ adicionales
9.3.3 Menú Fuente de datos
Atributos de entidades espaciales y tabla
Abrir tabla de atributos en la ventana adosada (requiere reiniciar QGIS)
Copiar geometría en WKT representación de la tabla de atributos. Al utilizar :sup:‘ Copiar las filas
seleccionadas al portapapeles‘ desde el diálogo Tabla de atributos, este tiene el resultado que las coorde-
nadas de los puntos o vértices también se copian en el portapapeles.
Funcionamiento de la tabla de atributos . Hay tres posibilidades: ‘Mostrar todos los objetos espa-
ciales’, ‘Mostrar objetos seleccionados’ y ‘Mostrar objetos espaciales visibles en el mapa’.
Caché de registro de tabla de atributos . Esta fila en caché hace posible guardar la última carga de
N filas de atributos de modo que el trabajo con la tabla de atributos será más rápido. El caché se borrará
cuando cierre la tabla de atributos.
Representación de valores NULOS. Aquí, puede definir un valor para los datos de campos que tienen un
valor NULO.
Manejo de fuente de datos
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Figure 9.3: Variables de entorno del sistema en QGIS
Buscar elementos válidos en el dock del explorador . Puede elegir entre ‘Comprobar extensión’ y
‘Comprobar contenido de archivo’.
Analizar en busca de contenido de archivos comprimidos (zip) en navegador base . ‘No’, ‘Explo-
ración básica’ y ‘Exploración completa’ son posibles.
Solicitar subcapas raster al abrir. Algunas subcapas raster soportadas — se les llama subdataset en GDAL.
Un ejemplo son los archivos netCDF — si hay muchos variables netCDF, GDAL ve cada variable como un
subconjunto de datos. La opción le permite controlar cómo lidiar con subcapas cuando se abre un archivo
con subcapas. Dispone de las siguientes opciones:
‘Siempre’: Siempre preguntar (Si hay subcapas existentes)
‘Si es necesario’: Preguntar si la capa no tiene bandas, pero tiene subcapas
‘Nunca’: Nunca preguntar, no se cargará nada
‘Cargar todo’: Nunca preguntar, pero cargar todas las subcapas
Ignorar la declaración de codificación del archivo shape. Si el archivo shape tiene información de
codificación, Este será ignorado por QGIS.
Añadir capas PostGIS con doble clic y seleccionar en modo extendido
Añadir capas de Oracle con doble clic y seleccionar en modo extendido
9.3.4 Menú representación
Comportamiento de presentación
Por defecto las nuevas capas añadidas al mapa se deben mostrar
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QGIS User Guide, Publicación 2.8
Utilizar el cacheo de presentación en lo posible a la velocidad de regeneración
Representación de capas en paralelo utilizando muchos núcleos CPU
Máximo de núcleos a utilizar
Intervalo de actualización del mapa (por defecto 250 ms)
Habilitar simplificación de objetos espaciales por defecto a las nuevas capas añadidas
Simplificación del umbral
Simplifique el lado del proveedor si es posible
Escala máxima a la que la capa se debe simplificar
Calidad de representación
Hacer que las líneas se muestren menos quebradas a expensas del rendimiento de la representación
Rásters
Con Selección de la banda RGB, puede definir el numero para la banda Roja, Verde y Azul.
Mejora de contraste
Unibanda gris . Una sola banda de gris puede tener ‘Sin realce’, ‘Estirar a MinMax’, ‘Estirar y cortar
a MinMax’ y también ‘Cortar a MinMax’.
Color de multibanda (byte/band) . Las opciones son ‘Sin realce’, ‘Estirar a MinMax’, ‘Estirar y cortar
a MinMax’ y ‘Cortar a MinMax’.
Color de multibanda (>byte/band) . Las opciones son ‘No realce’, ‘Estirar a MinMax’, ‘Estirar y
cortar a MinMax’ y ‘Cortar a MinMax’.
Límites (mínimo/máximo) . Las opciones son ‘Corte del conteo acumulativo’, ‘Min/Máx’, ‘Media +/-
desviación estándar’.
Límite para corte del conteo acumulativo de píxeles
Multiplicador de la desviación estándar
Depuración
Refrescar lienzo de mapa
9.3.5 Menú de Colores
Este menú le permite añadir algunos colores personalizados que puede encontrar en cada ventana de diálogo de
color de la representación. Verá un conjunto de colores predefinidos en la pestaña: se puede eliminar o editar
todos ellos. Por otra parte se puede añadir el color que se desea y realizar algunas operaciones de copiar y pegar.
Finalmente se puede exportar el conjunto de colores como un archivo gpl o importarlos.
9.3.6 Menú Vista del mapa y leyenda
Apariencia del mapa predeterminado (anulado por las propiedades del proyecto)
Definir un Color de selección y un Color de fondo.
Leyenda de capa
Acción doble clic en la leyenda . Puede ‘Abrir las propiedades de la capa’ o ‘Abrir la tabla de atributos’
con el doble clic.
Lo siguiente es posible Estilos de elementos de la leyenda:
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Comenzar el nombre de las capas con mayúsculas
Poner en negrita los nombres de la capa
Poner en negrita los nombres de grupo
Mostrar nombres de atributos de clasificación
Crear iconos de ráster (puede ser lento)
9.3.7 Menú Herramientas de mapa
Este menú ofrece algunas opciones con respecto al funcionamiento de la Herramienta de Identificación.
Radio de búsqueda para identificar y visualizar avisos en el mapa es un factor de tolerancia expresada
como un porcentaje del ancho de mapa. Esto significa que la herramienta de identificación representara los
resultados siempre y cuando haga clic dentro de esta tolerancia.
Color de realce le permite elegir con que color deben ser identificados los objetos espaciales que están
resaltados.
Buffer expresado como un porcentaje de el ancho del mapa, determina una distancia de separación que se
representa a partir del contorno de lo mas destacado a identificar.
El ancho mínimo expresado como un porcentaje de la anchura del mapa, determina el grosor del contorno
de cómo debe ser un objeto resaltado.
Herramienta de medición
Definir Color de la banda de medida para herramienta de medida
Definir Lugares decimales
Mantener unidad base
Unidades de medida preferidas (‘Metros’, ‘Pies’, ‘Millas náuticas’ o ‘Grados’)‘
Unidades de ángulos preferidas (‘Grados’, ‘Radianes’ o ‘Grados centecimales’)
Mover y zum
Definir Acción de la rueda del ratón (‘Zum’, ‘Zum y centrar’, ‘Zoom al cursor del ratón’, ‘Nada’)
Definir Factor de zum para la rueda del ratón
Escalas predefinidas
Aquí, encontrará una liste de escalas predefinidas. Con los botones [+] y[-] puede añadir o eliminar las escalas
individuales.
9.3.8 Menú Diseñador
Predeterminados de la composición
Se puede definir el Tipo de letra predeterminado aquí.
Apariencia de la cuadrícula
Definir el Estilo de cuadrícula (‘Sólido, ‘Puntos’, ‘Cruces’)
Definir el Color de la cuadrícula
Valores predeterminados de la cuadrícula y guía
Definir la Separc
Definir el Desplazamiento de cuadrícula para x y y
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Definir la Tolerancia de Ajuste
9.3.9 Menú Digitalización
Creación de entidades espaciales
Suprimir formulario emergente de atributos después de crear objetos espaciales
Reutilizar últimos valores de atributos introducidos
Validar geometrías. Editar lineas y polígonos complejos con muchos nodos puede resultar a una repre-
sentación muy lenta. Esto se debe a los procesos de validación por defecto en QGIS puede tomar mucho
tiempo. Para acelerar la representación, es posible seleccionar la validación de geometría GEOS (a partir de
GEOS 3.3) o a pagarlo. La validación de geometría GEOS es mucho más rápido, pero la desventaja es que
sólo el primer problema de geometría será reportado.
Banda de medición
Definir banda elástica Ancho de línea yColor de línea
Autoensamblado
Abrir opciones de autoensamblado en una ventana adosada(requiere reiniciar QGIS)
Definir Modo de autoensamblado por omisión (‘A vértice’, A segmento’, ‘A vértice y segmento’,
‘Desconectado’)
Definir Tolerancia de autoensamblado predeterminado en unidades de mapa o píxeles
Definir el Radio de búsqueda para edición de vértices en unidades de mapa o píxeles
Marcar vértices
Mostrar marcadores sólo para los objetos espaciales seleccionados
Definir vértice Estilo de marcador (‘Cruz’ (predeterminado), ‘Círculo semitransparente’ o ‘Nada’)
Definir vértice Tamaño de marcador
Herramienta de desplazamiento de curva
Las siguientes 3 opciones se refieren a la herramienta Desplazar curva en Advanced digitizing. A través de las
diversas configuraciones, es posible influir en la forma del desplazamiento de la línea. Estas opciones son posibles
a partir de GEOS 3.3.
Estilo de la unión
Segmentos del cuadrante
Límite Miter
9.3.10 Menú GDAL
GDAL es una biblioteca de intercambio de datos para archivos ráster. Es esta pestaña, puede Editar opciones de
creación yEditar opciones de pirámides de los formatos ráster. Definir que controlador GDAL se va a utilizar
para un formato ráster, como en algunos casos más de un controlador está disponible.
9.3.11 Menú SRC
SRC predeterminado para nuevos proyectos
No habilitar la reproyección ‘al vuelo’
Habilitar automáticamente la reproyección al vuelo si las capas tienen un SRC diferente
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Activar reproyección al vuelo por defecto
Seleccionar un SRC y Empezar siempre nuevos proyectos con este SRC
SRC para nuevas capas
Esta área permite definir la acción a realizar cuando una nueva capa es creada, o cuando una capa sin SRC es
cargada.
Solicitar SRC
Usar SRC del proyecto
Usar el SRC predeterminado
Por defecto transformación de datum
Preguntar por la trasformación del datum cuando el predeterminado no este definido
Si ha trabajado con la trasformación de SRC ‘al vuelo’ puede ver el resultado de la transformación en la
ventana de abajo. Puede encontrar información acerca de ‘Origen SRC’ y ‘Destino SRC’ así como también
‘Transformación de datum de origen’ y ‘Trasformación de datum destino’.
9.3.12 Menú Idioma
Ignorar el idioma del sistema yIdioma a usar en su lugar
Información acerca del idioma del sistema
9.3.13 Menú Red
General
Definir Dirección de búsqueda de WMS, por omisión es http://geopole.org/wms/search?search=\%1\&type=rss
Definir Expiró el tiempo para solicitudes de red - por omisión 60000
Definir Periodo de expiración predeterminada para teselas WMS-C/WMTS (en horas) - por omisión 24
Definir Reintentar al máximo en caso de errores en la solicitud de tile
Definir Agente- Usuario
Configuración de caché
Definir la configuración del caché Directorio y un Tamaño.
Usar proxy para acceso web y definir ‘Servidor’, ‘Puerto’, ‘Usuario’, y ‘Contraseña’.
Establecer el Tipo de proxy de acuerdo a sus necesidades.
Default Proxy: Proxy se determina con base en el proxy de aplicación que establece el uso
Socks5Proxy: Proxy genérico para cualquier tipo de conexión. Soporta TCP, UDP, unión a un puerto
(conexiones entrantes) y autenticación.
HttpProxy: Implementado con el comando “CONNECT”, sólo admite conexiones TCP salientes; ad-
mite la autenticación.
HttpCachingProxy: Implementando el uso de comandos HTTP normales, es útil sólo en el contexto
de peticiones HTTP.
FtpCachingProxy: Implementar el uso de un proxy FTP, es útil sólo en el contexto de las peticiones
FTP.
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Figure 9.4: Configurar proxy en QGIS
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Excluir algunas URLs se puede agregar a la caja de texto debajo los valores del proxy (ver Figure_Network_Tab).
Si necesita más información detallada acerca de las diferentes configuraciones de proxy, consulte el manual de
documentación de la biblioteca QT en http://doc.trolltech.com/4.5/qnetworkproxy.html#ProxyType-enum.
Truco: Utilizar proxies
El uso de proxies a veces puede ser complicado. Es útil para proceder por ‘prueba y error’ con los tipos de proxies
anteriores, comprobar para ver si en su caso tiene éxito.
Puede modificar las opciones de acuerdo a sus necesidades. Alguno de los cambios puede requerir un reinicio de
QGIS antes de hacerse efectivos.
La configuración se guarda en un archivo de texto: $HOME/.config/QGIS/QGIS2.conf
Puede encontrar sus ajustes en: $HOME/Library/Preferences/org.qgis.qgis.plist
Los ajustes se almacenan bajo el registro: HKEY\CURRENT_USER\Software\QGIS\qgis
9.4 Personalización
Las herramientas personalizadas permite que (des)active casi todos los elementos en la interfaz de usuario de
QGIS. Esto puede ser muy útil si se tienen muchos complementos instalados que nunca se utilizan y que esta
llenando su pantalla.
Figure 9.5: El diálogo de Personalización
La personalización de QGIS se divide en cinco grupos. En Menús, se pueden ocultar las entradas en la barra de
Menú. En Paneles se encuentran las ventanas de panel. Las ventanas del panel son aplicaciones que se pueden
iniciar y utilizar como una ventana flotante, de nivel superior o incrustadas en la ventana principal de QGIS como
un widget acoplado (ver también Paneles y Barras de Herramientas). En la Barra de estado, los elementos
como la información de coordenadas se puede desactivar. En las Barra de herramientas se puede (des)activar
los iconos de la barra de herramientas de QGIS, y en Widgets, se puede (des)activar los diálogos así como sus
botones.
Con Cambiar a la captura de widgets en la aplicación principal, puede hacer clic en los elementos en QGIS que desee que se
oculte y busque las entradas correspondientes en la personalización (ver figure_customization). También puede
guardar sus diferentes configuraciones para diferentes casos de uso. Antes de aplicar los cambios es necesario
reiniciar QGIS.
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CHAPTER 10
Working with Projections
QGIS allows users to define a global and project-wide CRS (coordinate reference system) for layers without a
pre-defined CRS. It also allows the user to define custom coordinate reference systems and supports on-the-fly
(OTF) projection of vector and raster layers. All of these features allow the user to display layers with different
CRSs and have them overlay properly.
10.1 Overview of Projection Support
QGIS has support for approximately 2,700 known CRSs. Definitions for each CRS are stored in a SQLite database
that is installed with QGIS. Normally, you do not need to manipulate the database directly. In fact, doing so may
cause projection support to fail. Custom CRSs are stored in a user database. See section Custom Coordinate
Reference System for information on managing your custom coordinate reference systems.
The CRSs available in QGIS are based on those defined by the European Petroleum Search Group (EPSG) and
the Institut Geographique National de France (IGNF) and are largely abstracted from the spatial reference tables
used in GDAL. EPSG identifiers are present in the database and can be used to specify a CRS in QGIS.
In order to use OTF projection, either your data must contain information about its coordinate reference system or
you will need to define a global, layer or project-wide CRS. For PostGIS layers, QGIS uses the spatial reference
identifier that was specified when the layer was created. For data supported by OGR, QGIS relies on the presence
of a recognized means of specifying the CRS. In the case of shapefiles, this means a file containing the well-known
text (WKT) specification of the CRS. This projection file has the same base name as the shapefile and a .prj
extension. For example, a shapefile named alaska.shp would have a corresponding projection file named
alaska.prj.
Whenever you select a new CRS, the layer units will automatically be changed in the General tab of the Project
Properties dialog under the Project (Gnome, OS X) or Settings (KDE, Windows) menu.
10.2 Global Projection Specification
QGIS starts each new project using the global default projection. The global default CRS is EPSG:4326 - WGS 84
(proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs), and it comes predefined in QGIS. This
default can be changed via the [Select...] button in the first section, which is used to define the default coordinate
reference system for new projects, as shown in figure_projection_1. This choice will be saved for use in subsequent
QGIS sessions.
When you use layers that do not have a CRS, you need to define how QGIS responds to these layers. This can be
done globally or project-wide in the CRS tab under Settings Options.
The options shown in figure_projection_1 are:
Prompt for CRS
Use project CRS
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Figure 10.1: CRS tab in the QGIS Options Dialog
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Use default CRS displayed below
If you want to define the coordinate reference system for a certain layer without CRS information, you can also do
that in the General tab of the raster and vector properties dialog (see General Menu for rasters and General Menu
for vectors). If your layer already has a CRS defined, it will be displayed as shown in Vector Layer Properties
Dialog .
Truco: CRS in the Map Legend
Right-clicking on a layer in the Map Legend (section Leyenda del mapa) provides two CRS shortcuts. Set layer
CRS takes you directly to the Coordinate Reference System Selector dialog (see figure_projection_2). Set project
CRS from Layer redefines the project CRS using the layer’s CRS.
10.3 Define On The Fly (OTF) Reprojection
QGIS supports OTF reprojection for both raster and vector data. However, OTF is not activated by default. To use
OTF projection, you must activate the Enable on the fly CRS transformation checkbox in the CRS tab of the
Project Properties dialog.
There are three ways to do this:
1. Select Project Properties from the Project (Gnome, OSX) or Settings (KDE, Windows) menu.
2. Click on the CRS status icon in the lower right-hand corner of the status bar.
3. Turn OTF on by default in the CRS tab of the Options dialog by selecting Enable ‘on the fly’ reprojection
by default or Automatically enable ‘on the fly’ reprojection if layers have different CRS.
If you have already loaded a layer and you want to enable OTF projection, the best practice is to open the CRS
tab of the Project Properties dialog, select a CRS, and activate the Enable ‘on the fly’ CRS transformation
checkbox. The CRS status icon will no longer be greyed out, and all layers will be OTF projected to the CRS
shown next to the icon.
The CRS tab of the Project Properties dialog contains five important components, as shown in Figure_projection_2
and described below:
1. Enable ‘on the fly’ CRS transformation — This checkbox is used to enable or disable OTF projection.
When off, each layer is drawn using the coordinates as read from the data source, and the components de-
scribed below are inactive. When on, the coordinates in each layer are projected to the coordinate reference
system defined for the map canvas.
2. Filter — If you know the EPSG code, the identifier, or the name for a coordinate reference system, you can
use the search feature to find it. Enter the EPSG code, the identifier or the name.
3. Recently used coordinate reference systems — If you have certain CRSs that you frequently use in your
everyday GIS work, these will be displayed in this list. Click on one of these items to select the associated
CRS.
4. Coordinate reference systems of the world — This is a list of all CRSs supported by QGIS, including
Geographic, Projected and Custom coordinate reference systems. To define a CRS, select it from the list by
expanding the appropriate node and selecting the CRS. The active CRS is preselected.
5. PROJ.4 text — This is the CRS string used by the PROJ.4 projection engine. This text is read-only and
provided for informational purposes.
Truco: Project Properties Dialog
If you open the Project Properties dialog from the Project menu, you must click on the CRS tab to view the CRS
settings.
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Figure 10.2: Project Properties Dialog
Opening the dialog from the CRS status icon will automatically bring the CRS tab to the front.
10.4 Custom Coordinate Reference System
If QGIS does not provide the coordinate reference system you need, you can define a custom CRS. To define a
CRS, select Custom CRS... from the Settings menu. Custom CRSs are stored in your QGIS user database. In
addition to your custom CRSs, this database also contains your spatial bookmarks and other custom data.
Defining a custom CRS in QGIS requires a good understanding of the PROJ.4 projection library. To begin, refer to
“Cartographic Projection Procedures for the UNIX Environment - A User’s Manual” by Gerald I. Evenden, U.S.
Geological Survey Open-File Report 90-284, 1990 (available at ftp://ftp.remotesensing.org/proj/OF90-284.pdf).
This manual describes the use of the proj.4 and related command line utilities. The cartographic parameters
used with proj.4 are described in the user manual and are the same as those used by QGIS.
The Custom Coordinate Reference System Definition dialog requires only two parameters to define a user CRS:
1. A descriptive name
2. The cartographic parameters in PROJ.4 format
To create a new CRS, click the Add new CRS button and enter a descriptive name and the CRS parameters.
Note that the Parameters must begin with a +proj= block, to represent the new coordinate reference system.
You can test your CRS parameters to see if they give sane results. To do this, enter known WGS 84 latitude and
longitude values in North and East fields, respectively. Click on [Calculate], and compare the results with the
known values in your coordinate reference system.
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Figure 10.3: Custom CRS Dialog
10.5 Default datum transformations
OTF depends on being able to transform data into a ‘default CRS’, and QGIS uses WGS84. For some CRS there
are a number of transforms available. QGIS allows you to define the transformation used otherwise QGIS uses a
default transformation.
In the CRS tab under Settings Options you can:
set QGIS to ask you when it needs define a transformation using Ask for datum transformation when no
default is defined
edit a list of user defaults for transformations.
QGIS asks which transformation to use by opening a dialogue box displaying PROJ.4 text describing the source
and destination transforms. Further information may be found by hovering over a transform. User defaults can be
saved by selecting Remember selection.
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CHAPTER 11
QGIS Browser
The QGIS Browser is a panel in QGIS that lets you easily navigate in your filesystem and manage geodata. You
can have access to common vector files (e.g., ESRI shapefiles or MapInfo files), databases (e.g., PostGIS, Oracle,
SpatiaLite or MS SQL Spatial) and WMS/WFS connections. You can also view your GRASS data (to get the data
into QGIS, see GRASS GIS Integration).
Figure 11.1: QGIS browser as a stand alone application
Use the QGIS Browser to preview your data. The drag-and-drop function makes it easy to get your data into the
map view and the map legend.
1. Activate the QGIS Browser: Right-click on the toolbar and check Browser or select it from Settings
Panels.
2. Drag the panel into the legend window and release it.
3. Click on the Browser tab.
4. Browse in your filesystem and choose the shapefile folder from qgis_sample_data directory.
5. Press the Shift key and select the airports.shp and alaska.shp files.
6. Press the left mouse button, then drag and drop the files into the map canvas.
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7. Right-click on a layer and choose Set project CRS from layer. For more information see Working with
Projections.
8. Click on Zoom Full to make the layers visible.
There is a second browser available under Settings Panels. This is handy when you need to move files or layers
between locations.
1. Activate a second QGIS Browser: Right-click on the toolbar and check Browser (2), or select it from
Settings Panels.
2. Drag the panel into the legend window.
3. Navigate to the Browser (2) tab and browse for a shapefile in your file system.
4. Select a file with the left mouse button. Now you can use the Add Selected Layers icon to add it into the
current project.
QGIS automatically looks for the coordinate reference system (CRS) and zooms to the layer extent if you work
in a blank QGIS project. If there are already files in your project, the file will just be added, and in the case that
it has the same extent and CRS, it will be visualized. If the file has another CRS and layer extent, you must first
right-click on the layer and choose Set Project CRS from Layer. Then choose Zoom to Layer Extent.
The Filter files function works on a directory level. Browse to the folder where you want to filter files and enter
a search word or wildcard. The Browser will show only matching filenames – other data won’t be displayed.
It’s also possible to run the QGIS Browser as a stand-alone application.
Start the QGIS browser
Type in “qbrowser” at a command prompt.
Start the QGIS Browser using the Start menu or desktop shortcut.
The QGIS Browser is available from your Applications folder.
In figure_browser_standalone_metadata, you can see the enhanced functionality of the stand-alone QGIS Browser.
The Param tab provides the details of your connection-based datasets, like PostGIS or MSSQL Spatial. The
Metadata tab contains general information about the file (see Metadata Menu). With the Preview tab, you can
have a look at your files without importing them into your QGIS project. It’s also possible to preview the attributes
of your files in the Attributes tab.
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CHAPTER 12
Trabajar con catos vectoriales
.
12.1 Supported Data Formats
QGIS uses the OGR library to read and write vector data formats, including ESRI shapefiles, MapInfo and Mi-
croStation file formats, AutoCAD DXF, PostGIS, SpatiaLite, Oracle Spatial and MSSQL Spatial databases, and
many more. GRASS vector and PostgreSQL support is supplied by native QGIS data provider plugins. Vector
data can also be loaded in read mode from zip and gzip archives into QGIS. As of the date of this document, 69
vector formats are supported by the OGR library (see OGR-SOFTWARE-SUITE in Referencias bibliográficas y
web). The complete list is available at http://www.gdal.org/ogr/ogr_formats.html.
Nota: Not all of the listed formats may work in QGIS for various reasons. For example, some require external
commercial libraries, or the GDAL/OGR installation of your OS may not have been built to support the format
you want to use. Only those formats that have been well tested will appear in the list of file types when loading a
vector into QGIS. Other untested formats can be loaded by selecting *.*.
Working with GRASS vector data is described in Section GRASS GIS Integration.
This section describes how to work with several common formats: ESRI shapefiles, PostGIS layers, SpatiaLite
layers, OpenStreetMap vectors, and Comma Separated data (CSV). Many of the features available in QGIS work
the same, regardless of the vector data source. This is by design, and it includes the identify, select, labeling and
attributes functions.
12.1.1 ESRI Shapefiles
The standard vector file format used in QGIS is the ESRI shapefile. Support is provided by the OGR Simple
Feature Library (http://www.gdal.org/ogr/).
A shapefile actually consists of several files. The following three are required:
1. .shp file containing the feature geometries
2. .dbf file containing the attributes in dBase format
3. .shx index file
Shapefiles also can include a file with a .prj suffix, which contains the projection information. While it is very
useful to have a projection file, it is not mandatory. A shapefile dataset can contain additional files. For further
details, see the ESRI technical specification at http://www.esri.com/library/whitepapers/pdfs/shapefile.pdf.
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Loading a Shapefile
To load a shapefile, start QGIS and click on the Add Vector Layer toolbar button, or simply press Ctrl+Shift+V.
This will bring up a new window (see figure_vector_1).
Figure 12.1: Add Vector Layer Dialog
From the available options check File. Click on [Browse]. That will bring up a standard open file dialog (see
figure_vector_2), which allows you to navigate the file system and load a shapefile or other supported data source.
The selection box Filter allows you to preselect some OGR-supported file formats.
You can also select the encoding for the shapefile if desired.
Figure 12.2: Open an OGR Supported Vector Layer Dialog
Selecting a shapefile from the list and clicking [Open] loads it into QGIS. Figure_vector_3 shows QGIS after
loading the alaska.shp file.
Truco: Layer Colors
When you add a layer to the map, it is assigned a random color. When adding more than one layer at a time,
different colors are assigned to each layer.
Once a shapefile is loaded, you can zoom around it using the map navigation tools. To change the style of a layer,
open the Layer Properties dialog by double clicking on the layer name or by right-clicking on the name in the
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Figure 12.3: QGIS with Shapefile of Alaska loaded
legend and choosing Properties from the context menu. See section Style Menu for more information on setting
symbology of vector layers.
Truco: Load layer and project from mounted external drives on OS X
On OS X, portable drives that are mounted beside the primary hard drive do not show up as expected under File
Open Project. We are working on a more OSX-native open/save dialog to fix this. As a workaround, you can
type /Volumes in the File name box and press Enter. Then you can navigate to external drives and network
mounts.
Improving Performance for Shapefiles
To improve the performance of drawing a shapefile, you can create a spatial index. A spatial index will improve
the speed of both zooming and panning. Spatial indexes used by QGIS have a .qix extension.
Use these steps to create the index:
Load a shapefile by clicking on the Add Vector Layer toolbar button or pressing Ctrl+Shift+V.
Open the Layer Properties dialog by double-clicking on the shapefile name in the legend or by right-clicking
and choosing Properties from the context menu.
In the General tab, click the [Create Spatial Index] button.
Problem loading a shape .prj file
If you load a shapefile with a .prj file and QGIS is not able to read the coordinate reference system from that
file, you will need to define the proper projection manually within the General tab of the Layer Properties dialog
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of the layer by clicking the [Specify...] button. This is due to the fact that .prj files often do not provide the
complete projection parameters as used in QGIS and listed in the CRS dialog.
For the same reason, if you create a new shapefile with QGIS, two different projection files are created: a .prj
file with limited projection parameters, compatible with ESRI software, and a .qpj file, providing the complete
parameters of the used CRS. Whenever QGIS finds a .qpj file, it will be used instead of the .prj.
12.1.2 Loading a MapInfo Layer
To load a MapInfo layer, click on the Add Vector Layer toolbar button; or type Ctrl+Shift+V, change the
file type filter Files of type : to ‘Mapinfo File [OGR] (*.mif *.tab *.MIF *.TAB)’ and select the MapInfo
layer you want to load.
12.1.3 Loading an ArcInfo Binary Coverage
To load an ArcInfo Binary Coverage, click on the Add Vector Layer toolbar button or press Ctrl+Shift+V
to open the Add Vector Layer dialog. Select Directory as Source type. Change the file type filter Files of type
to ‘Arc/Info Binary Coverage’. Navigate to the directory that contains the coverage file, and select it.
Similarly, you can load directory-based vector files in the UK National Transfer Format, as well as the raw TIGER
Format of the US Census Bureau.
12.1.4 Delimited Text Files
Tabular data is a very common and widely used format because of its simplicity and readability – data can be
viewed and edited even in a plain text editor. A delimited text file is an attribute table with each column separated
by a defined character and each row separated by a line break. The first row usually contains the column names. A
common type of delimited text file is a CSV (Comma Separated Values), with each column separated by a comma.
Such data files can also contain positional information in two main forms:
As point coordinates in separate columns
As well-known text (WKT) representation of geometry
QGIS allows you to load a delimited text file as a layer or ordinal table. But first check that the file meets the
following requirements:
1. The file must have a delimited header row of field names. This must be the first line in the text file.
2. The header row must contain field(s) with geometry definition. These field(s) can have any name.
3. The X and Y coordinates (if geometry is defined by coordinates) must be specified as numbers. The coor-
dinate system is not important.
As an example of a valid text file, we import the elevation point data file elevp.csv that comes with the QGIS
sample dataset (see section Datos de ejemplo):
X;Y;ELEV
-300120;7689960;13
-654360;7562040;52
1640;7512840;3
[...]
Some items to note about the text file:
1. The example text file uses ;(semicolon) as delimiter. Any character can be used to delimit the fields.
2. The first row is the header row. It contains the fields X,Yand ELEV.
3. No quotes (") are used to delimit text fields.
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4. The X coordinates are contained in the Xfield.
5. The Y coordinates are contained in the Yfield.
Loading a delimited text file
Click the toolbar icon Add Delimited Text Layer in the Manage layers toolbar to open the Create a Layer from a
Delimited Text File dialog, as shown in figure_delimited_text_1.
Figure 12.4: Delimited Text Dialog
First, select the file to import (e.g., qgis_sample_data/csv/elevp.csv) by clicking on the [Browse]
button. Once the file is selected, QGIS attempts to parse the file with the most recently used delimiter. To
enable QGIS to properly parse the file, it is important to select the correct delimiter. You can specify a delimiter
by activating Custom delimiters, or by activating Regular expression delimiter and entering text into the
Expression field. For example, to change the delimiter to tab, use \t (this is a regular expression for the tab
character).
Once the file is parsed, set Geometry definition to Point coordinates and choose the Xand Yfields from the
dropdown lists. If the coordinates are defined as degrees/minutes/seconds, activate the DMS coordinates
checkbox.
Finally, enter a layer name (e.g., elevp), as shown in figure_delimited_text_1. To add the layer to the map, click
[OK]. The delimited text file now behaves as any other map layer in QGIS.
There is also a helper option that allows you to trim leading and trailing spaces from fields — Trim fields.
Also, it is possible to Discard empty fields. If necessary, you can force a comma to be the decimal separator
by activating Decimal separator is comma.
If spatial information is represented by WKT, activate the Well Known Text option and select the field with the
WKT definition for point, line or polygon objects. If the file contains non-spatial data, activate No geometry
(attribute only table) and it will be loaded as an ordinal table.
Additionaly, you can enable:
Use spatial index to improve the performance of displaying and spatially selecting features.
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Use subset index.
Watch file to watch for changes to the file by other applications while QGIS is running.
12.1.5 OpenStreetMap data
In recent years, the OpenStreetMap project has gained popularity because in many countries no free geodata such
as digital road maps are available. The objective of the OSM project is to create a free editable map of the world
from GPS data, aerial photography or local knowledge. To support this objective, QGIS provides suppport for
OSM data.
Loading OpenStreetMap Vectors
QGIS integrates OpenStreetMap import as a core functionality.
To connect to the OSM server and download data, open the menu Vector Openstreetmap Load data.
You can skip this step if you already obtained an .osm XML file using JOSM, Overpass API or any other
source.
The menu Vector Openstreetmap Import topology from an XML file will convert your .osm file into
a SpatiaLite database and create a corresponding database connection.
The menu Vector Openstreetmap Export topology to SpatiaLite then allows you to open the database
connection, select the type of data you want (points, lines, or polygons) and choose tags to import. This
creates a SpatiaLite geometry layer that you can add to your project by clicking on the Add SpatiaLite Layer
toolbar button or by selecting the Add SpatiaLite Layer... option from the Layer menu (see section
SpatiaLite Layers).
12.1.6 PostGIS Layers
PostGIS layers are stored in a PostgreSQL database. The advantages of PostGIS are the spatial indexing, filter-
ing and query capabilities it provides. Using PostGIS, vector functions such as select and identify work more
accurately than they do with OGR layers in QGIS.
Creating a stored Connection
The first time you use a PostGIS data source, you must create a connection to the PostgreSQL database that
contains the data. Begin by clicking on the Add PostGIS Layer toolbar button, selecting the Add PostGIS
Layer... option from the Layer menu, or typing Ctrl+Shift+D. You can also open the Add Vector Layer dialog
and select Database. The Add PostGIS Table(s) dialog will be displayed. To access the connection manager,
click on the [New] button to display the Create a New PostGIS Connection dialog. The parameters required for a
connection are:
Name: A name for this connection. It can be the same as Database.
Service: Service parameter to be used alternatively to hostname/port (and potentially database). This can
be defined in pg_service.conf.
Host: Name of the database host. This must be a resolvable host name such as would be used to open a
telnet connection or ping the host. If the database is on the same computer as QGIS, simply enter ‘localhost’
here.
Port: Port number the PostgreSQL database server listens on. The default port is 5432.
Database: Name of the database.
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SSL mode: How the SSL connection will be negotiated with the server. Note that massive speedups in
PostGIS layer rendering can be achieved by disabling SSL in the connection editor. The following options
are available:
Disable: Only try an unencrypted SSL connection.
Allow: Try a non-SSL connection. If that fails, try an SSL connection.
Prefer (the default): Try an SSL connection. If that fails, try a non-SSL connection.
Require: Only try an SSL connection.
Username: User name used to log in to the database.
Password: Password used with Username to connect to the database.
Optionally, you can activate the following checkboxes:
Save Username
Save Password
Only look in the geometry_columns table
Don’t resolve type of unrestricted columns (GEOMETRY)
Only look in the ‘public’ schema
Also list tables with no geometry
Use estimated table metadata
Once all parameters and options are set, you can test the connection by clicking on the [Test Connect] button.
Loading a PostGIS Layer
Once you have one or more connections defined, you can load layers from the PostgreSQL database. Of
course, this requires having data in PostgreSQL. See section Importing Data into PostgreSQL for a discussion on
importing data into the database.
To load a layer from PostGIS, perform the following steps:
If the Add PostGIS layers dialog is not already open, selecting the Add PostGIS Layer... option from the
Layer menu or typing Ctrl+Shift+D opens the dialog.
Choose the connection from the drop-down list and click [Connect].
Select or unselect Also list tables with no geometry.
Optionally, use some Search Options to define which features to load from the layer, or use the [Build
query] button to start the Query builder dialog.
Find the layer(s) you wish to add in the list of available layers.
Select it by clicking on it. You can select multiple layers by holding down the Shift key while clicking.
See section Constructor de consultas for information on using the PostgreSQL Query Builder to further
define the layer.
Click on the [Add] button to add the layer to the map.
Truco: PostGIS Layers
Normally, a PostGIS layer is defined by an entry in the geometry_columns table. From version 0.9.0 on, QGIS
can load layers that do not have an entry in the geometry_columns table. This includes both tables and views.
Defining a spatial view provides a powerful means to visualize your data. Refer to your PostgreSQL manual for
information on creating views.
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Some details about PostgreSQL layers
This section contains some details on how QGIS accesses PostgreSQL layers. Most of the time, QGIS should
simply provide you with a list of database tables that can be loaded, and it will load them on request. However,
if you have trouble loading a PostgreSQL table into QGIS, the information below may help you understand any
QGIS messages and give you direction on changing the PostgreSQL table or view definition to allow QGIS to
load it.
QGIS requires that PostgreSQL layers contain a column that can be used as a unique key for the layer. For tables,
this usually means that the table needs a primary key, or a column with a unique constraint on it. In QGIS, this
column needs to be of type int4 (an integer of size 4 bytes). Alternatively, the ctid column can be used as primary
key. If a table lacks these items, the oid column will be used instead. Performance will be improved if the column
is indexed (note that primary keys are automatically indexed in PostgreSQL).
If the PostgreSQL layer is a view, the same requirement exists, but views do not have primary keys or columns
with unique constraints on them. You have to define a primary key field (has to be integer) in the QGIS dialog
before you can load the view. If a suitable column does not exist in the view, QGIS will not load the layer. If this
occurs, the solution is to alter the view so that it does include a suitable column (a type of integer and either a
primary key or with a unique constraint, preferably indexed).
QGIS offers a checkbox Select at id that is activated by default. This option gets the ids without the attributes
which is faster in most cases. It can make sense to disable this option when you use expensive views.
Truco: Backup of PostGIS database with layers saved by QGIS
If you want to make a backup of your PostGIS database using the pg_dump and pg_restore commands
the default layer styles as saved by QGIS are failing to restore afterwards. You need to set the XML option to
DOCUMENT and the restore will work.
12.1.7 Importing Data into PostgreSQL
Data can be imported into PostgreSQL/PostGIS using several tools, including the SPIT plugin and the command
line tools shp2pgsql and ogr2ogr.
DB Manager
QGIS comes with a core plugin named DB Manager. It can be used to load shapefiles and other data formats, and
it includes support for schemas. See section Complemento administrador de BBDD for more information.
shp2pgsql
PostGIS includes an utility called shp2pgsql that can be used to import shapefiles into a PostGIS-enabled database.
For example, to import a shapefile named lakes.shp into a PostgreSQL database named gis_data, use the
following command:
shp2pgsql -s 2964 lakes.shp lakes_new | psql gis_data
This creates a new layer named lakes_new in the gis_data database. The new layer will have a spatial ref-
erence identifier (SRID) of 2964. See section Working with Projections for more information on spatial reference
systems and projections.
Truco: Exporting datasets from PostGIS
Like the import tool shp2pgsql, there is also a tool to export PostGIS datasets as shapefiles: pgsql2shp. This is
shipped within your PostGIS distribution.
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ogr2ogr
Besides shp2pgsql and DB Manager, there is another tool for feeding geodata in PostGIS: ogr2ogr. This is part
of your GDAL installation.
To import a shapefile into PostGIS, do the following:
ogr2ogr -f "PostgreSQL" PG:"dbname=postgis host=myhost.de user=postgres
password=topsecret" alaska.shp
This will import the shapefile alaska.shp into the PostGIS database postgis using the user postgres with the
password topsecret on host server myhost.de.
Note that OGR must be built with PostgreSQL to support PostGIS. You can verify this by typing (in )
ogrinfo --formats | grep -i post
If you prefer to use PostgreSQLs COPY command instead of the default INSERT INTO method, you can export
the following environment variable (at least available on and ):
export PG_USE_COPY=YES
ogr2ogr does not create spatial indexes like shp2pgsl does. You need to create them manually, using the nor-
mal SQL command CREATE INDEX afterwards as an extra step (as described in the next section Improving
Performance).
Improving Performance
Retrieving features from a PostgreSQL database can be time-consuming, especially over a network. You can
improve the drawing performance of PostgreSQL layers by ensuring that a PostGIS spatial index exists on
each layer in the database. PostGIS supports creation of a GiST (Generalized Search Tree) index to speed
up spatial searches of the data (GiST index information is taken from the PostGIS documentation available at
http://postgis.refractions.net).
The syntax for creating a GiST index is:
CREATE INDEX [indexname] ON [tablename]
USING GIST ( [geometryfield] GIST_GEOMETRY_OPS );
Note that for large tables, creating the index can take a long time. Once the index is created, you should perform
aVACUUM ANALYZE. See the PostGIS documentation (POSTGIS-PROJECT Referencias bibliográficas y web)
for more information.
The following is an example of creating a GiST index:
gsherman@madison:~/current$ psql gis_data
Welcome to psql 8.3.0, the PostgreSQL interactive terminal.
Type: \copyright for distribution terms
\h for help with SQL commands
\? for help with psql commands
\g or terminate with semicolon to execute query
\q to quit
gis_data=# CREATE INDEX sidx_alaska_lakes ON alaska_lakes
gis_data-# USING GIST (the_geom GIST_GEOMETRY_OPS);
CREATE INDEX
gis_data=# VACUUM ANALYZE alaska_lakes;
VACUUM
gis_data=# \q
gsherman@madison:~/current$
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12.1.8 Vector layers crossing 180° longitude
Many GIS packages don’t wrap vector maps with a geographic reference system (lat/lon) crossing the 180 degrees
longitude line (http://postgis.refractions.net/documentation/manual-2.0/ST_Shift_Longitude.html). As result, if
we open such a map in QGIS, we will see two far, distinct locations, that should appear near each other. In
Figure_vector_4, the tiny point on the far left of the map canvas (Chatham Islands) should be within the grid, to
the right of the New Zealand main islands.
Figure 12.5: Map in lat/lon crossing the 180° longitude line
A work-around is to transform the longitude values using PostGIS and the ST_Shift_Longitude function. This
function reads every point/vertex in every component of every feature in a geometry, and if the longitude coordi-
nate is < 0°, it adds 360° to it. The result is a 0° - 360° version of the data to be plotted in a 180°-centric map.
Figure 12.6: Crossing 180° longitude applying the ST_Shift_Longitude function
Usage
Import data into PostGIS (Importing Data into PostgreSQL) using, for example, the DB Manager plugin.
Use the PostGIS command line interface to issue the following command (in this example,
“TABLE” is the actual name of your PostGIS table): gis_data=# update TABLE set
the_geom=ST_Shift_Longitude(the_geom);
If everything went well, you should receive a confirmation about the number of features that were updated.
Then you’ll be able to load the map and see the difference (Figure_vector_5).
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12.1.9 SpatiaLite Layers
The first time you load data from a SpatiaLite database, begin by clicking on the Add SpatiaLite Layer
toolbar button, or by selecting the Add SpatiaLite Layer... option from the Layer menu, or by typing
Ctrl+Shift+L. This will bring up a window that will allow you either to connect to a SpatiaLite database
already known to QGIS, which you can choose from the drop-down menu, or to define a new connection to a new
database. To define a new connection, click on [New] and use the file browser to point to your SpatiaLite database,
which is a file with a .sqlite extension.
If you want to save a vector layer to SpatiaLite format, you can do this by right clicking the layer in the legend.
Then, click on Save as.., define the name of the output file, and select ‘SpatiaLite’ as format and the CRS. Also,
you can select ‘SQLite’ as format and then add SPATIALITE=YES in the OGR data source creation option field.
This tells OGR to create a SpatiaLite database. See also http://www.gdal.org/ogr/drv_sqlite.html.
QGIS also supports editable views in SpatiaLite.
Creating a new SpatiaLite layer
If you want to create a new SpatiaLite layer, please refer to section Creating a new SpatiaLite layer.
Truco: SpatiaLite data management Plugins
For SpatiaLite data management, you can also use several Python plugins: QSpatiaLite, SpatiaLite Manager or
DB Manager (core plugin, recommended). If necessary, they can be downloaded and installed with the Plugin
Installer.
12.1.10 MSSQL Spatial Layers
QGIS also provides native MS SQL 2008 support. The first time you load MSSQL Spatial data, begin by
clicking on the Add MSSQL Spatial Layer toolbar button or by selecting the Add MSSQL Spatial Layer... option
from the Layer menu, or by typing Ctrl+Shift+M.
12.1.11 Oracle Spatial Layers
The spatial features in Oracle Spatial aid users in managing geographic and location data in a native type within
an Oracle database. QGIS now has support for such layers.
Creating a stored Connection
The first time you use an Oracle Spatial data source, you must create a connection to the database that
contains the data. Begin by clicking on the Add Orcale Spatial Layer toolbar button, selecting the Add Orcale
Spatial Layer... option from the Layer menu, or typing Ctrl+Shift+O. To access the connection manager, click
on the [New] button to display the Create a New Oracle Spatial Connection dialog. The parameters required for
a connection are:
Name: A name for this connection. It can be the same as Database
Database: SID or SERVICE_NAME of the Oracle instance.
Host: Name of the database host. This must be a resolvable host name such as would be used to open a
telnet connection or ping the host. If the database is on the same computer as QGIS, simply enter ‘localhost’
here.
Port: Port number the Oracle database server listens on. The default port is 1521.
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Username: Username used to login to the database.
Password: Password used with Username to connect to the database.
Optionally, you can activate following checkboxes:
Save Username Indicates whether to save the database username in the connection configuration.
Save Password Indicates whether to save the database password in the connection settings.
Only look in meta data table Restricts the displayed tables to those that are in the
all_sdo_geom_metadata view. This can speed up the initial display of spatial tables.
Only look for user’s tables When searching for spatial tables, restrict the search to tables that are owned
by the user.
Also list tables with no geometry Indicates that tables without geometry should also be listed by default.
Use estimated table statistics for the layer metadata When the layer is set up, various metadata are
required for the Oracle table. This includes information such as the table row count, geometry type and
spatial extents of the data in the geometry column. If the table contains a large number of rows, determining
this metadata can be time-consuming. By activating this option, the following fast table metadata operations
are done: Row count is determined from all_tables.num_rows. Table extents are always determined
with the SDO_TUNE.EXTENTS_OF function, even if a layer filter is applied. Table geometry is determined
from the first 100 non-null geometry rows in the table.
Only existing geometry types Only list the existing geometry types and don’t offer to add others.
Once all parameters and options are set, you can test the connection by clicking on the [Test Connect] button.
Truco: QGIS User Settings and Security
Depending on your computing environment, storing passwords in your QGIS settings may be a security risk.
Passwords are saved in clear text in the system configuration and in the project files! Your customized settings for
QGIS are stored based on the operating system:
The settings are stored in your home directory in ~/.qgis2.
The settings are stored in the registry.
Loading an Oracle Spatial Layer
Once you have one or more connections defined, you can load layers from the Oracle database. Of course,
this requires having data in Oracle.
To load a layer from Oracle Spatial, perform the following steps:
If the Add Oracle Spatial layers dialog is not already open, click on the Add Oracle Spatial Layer toolbar
button.
Choose the connection from the drop-down list and click [Connect].
Select or unselect Also list tables with no geometry.
Optionally, use some Search Options to define which features to load from the layer or use the [Build
query] button to start the Query builder dialog.
Find the layer(s) you wish to add in the list of available layers.
Select it by clicking on it. You can select multiple layers by holding down the Shift key while clicking.
See section Constructor de consultas for information on using the Oracle Query Builder to further define
the layer.
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Click on the [Add] button to add the layer to the map.
Truco: Oracle Spatial Layers
Normally, an Oracle Spatial layer is defined by an entry in the USER_SDO_METADATA table.
.
12.2 The Symbol Library
12.2.1 Presentation
The Symbol Library is the place where users can create generic symbols to be used in several QGIS projects. It
allows users to export and import symbols, groups symbols and add, edit and remove symbols. You can open it
with the Settings Style Library or from the Style tab in the vector layer’s Properties.
Share and import symbols
Users can export and import symbols in two main formats: qml (QGIS format) and SLD (OGC standard). Note
that SLD format is not fully supported by QGIS.
share item displays a drop down list to let the user import or export symbols.
Groups and smart groups
Groups are categories of Symbols and smart groups are dynamic groups.
To create a group, right-click on an existing group or on the main Groups directory in the left of the library. You
can also select a group and click on the add item button.
To add a symbol into a group, you can either right click on a symbol then choose Apply group and then the group
name added before. There is a second way to add several symbols into group: just select a group and click
and choose Group Symbols. All symbols display a checkbox that allow you to add the symbol into the selected
groups. When finished, you can click on the same button, and choose Finish Grouping.
Create Smart Symbols is similar to creating group, but instead select Smart Groups. The dialog box allow user
to choose the expression to select symbols in order to appear in the smart group (contains some tags, member of
a group, have a string in its name, etc.)
Add, edit, remove symbol
With the Style manager from the [Symbol] menu you can manage your symbols. You can add item,
edit item,remove item and share item. ‘Marker’ symbols, ‘Line’ symbols, ‘Fill’ patterns and ‘colour ramps’
can be used to create the symbols. The symbols are then assigned to ‘All Symbols’, ‘Groups’ or ‘Smart groups’.
For each kind of symbols, you will find always the same dialog structure:
at the top left side a symbol representation
under the symbol representation the symbol tree show the symbol layers
at the right you can setup some parameter (unit,transparency, color, size and rotation)
under these parameters you find some symbol from the symbols library
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The symbol tree allow adding, removing or protect new simple symbol. You can move up or down the symbol
layer.
More detailed settings can be made when clicking on the second level in the Symbol layers dialog. You can define
Symbol layers that are combined afterwards. A symbol can consist of several Symbol layers. Settings will be
shown later in this chapter.
Truco: Note that once you have set the size in the lower levels of the Symbol layers dialog, the size of the
whole symbol can be changed with the Size menu in the first level again. The size of the lower levels changes
accordingly, while the size ratio is maintained.
12.2.2 Marker Symbols
Marker symbols have several symbol layer types:
Ellipse marker
Font marker
Simple marker (default)
SVG marker
Vector Field marker
The following settings are possible:
Symbol layer type: You have the option to use Ellipse markers, Font markers, Simple markers, SVG markers
and Vector Field markers.
colors
Size
Outline style
Outline width
Angle
Offset X,Y: You can shift the symbol in the x- or y-direction.
Anchor point
Data defined properties ...
12.2.3 Line Symbols
Line marker symbols have only two symbol layer types:
Marker line
Simple line (default)
The default symbol layer type draws a simple line whereas the other display a marker point regularly on the line.
You can choose different location vertex, interval or central point. Marker line can have offset along the line or
offset line. Finally, rotation allows you to change the orientation of the symbol.
The following settings are possible:
colour
Pen width
Offset
Pen style
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Join style
Cap style
Use custom dash pattern
Dash pattern unit
Data defined properties ...
12.2.4 Polygon Symbols
Polygon marker symbols have also several symbol layer types:
Centroid fill
Gradient fill
Line pattern fill
Point pattern fill
Raster image fill
SVG fill
Shapeburst fill
Simple fill (default)
Outline: Marker line (same as line marker)
Outline: simple line (same as line marker)
The following settings are possible:
Colors for the border and the fill.
Fill style
Border style
Border width
Offset X,Y
Data defined properties ...
Using the color combo box, you can drag and drop color for one color button to another button, copy-paste color,
pick color from somewhere, choose a color from the palette or from recent or standard color. The combo box
allow you to fill in the feature with transparency. You can also just click on the button to open the palettte dialog.
Note that you can import color from some external software like GIMP.
With the ‘Raster image fill’ you can fill polygons with a tiled raster image. Options include (data defined) file
name, opacity, image size (in pixels, mm or map units), coordinate mode (feature or view) and rotation.
‘Gradient Fill’ Symbol layer type allows you to select between a Two color and Color ramp setting. You
can use the Feature centroid as Referencepoint. All fills ‘Gradient Fill‘ Symbol layer type is also available
through the Symbol menu of the Categorized and Graduated Renderer and through the Rule properties menu of
the Rule-based renderer. Other possibility is to choose a ‘shapeburst fill’ which is a buffered gradient fill, where a
gradient is drawn from the boundary of a polygon towards the polygon’s centre. Configurable parameters include
distance from the boundary to shade, use of color ramps or simple two color gradients, optional blurring of the fill
and offsets.
It is possible to only draw polygon borders inside the polygon. Using ‘Outline: Simple line’ select Draw line
only inside polygon.
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12.2.5 Color ramp
You can create a custom color ramp choosing New color ramp... from the color ramp drop-down menu. A dialog
will prompt for the ramp type: Gradient, Random, colorBrewer, or cpt-city. The first three have options for number
of steps and/or multiple stops in the color ramp. You can use the Invert option while classifying the data with
a color ramp. See figure_symbology_3 for an example of custom color ramp and figure_symbology_3a for the
cpt-city dialog.
Figure 12.7: Example of custom gradient color ramp with multiple stops
The cpt-city option opens a new dialog with hundreds of themes included ‘out of the box’.
.
12.3 The Vector Properties Dialog
The Layer Properties dialog for a vector layer provides information about the layer, symbology settings and
labeling options. If your vector layer has been loaded from a PostgreSQL/PostGIS datastore, you can also alter
the underlying SQL for the layer by invoking the Query Builder dialog on the General tab. To access the Layer
Properties dialog, double-click on a layer in the legend or right-click on the layer and select Properties from the
pop-up menu.
12.3.1 Style Menu
The Style menu provides you with a comprehensive tool for rendering and symbolizing your vector data. You can
use Layer rendering tools that are common to all vector data, as well as special symbolizing tools that were
designed for the different kinds of vector data.
Renderers
The renderer is responsible for drawing a feature together with the correct symbol. There are four types of
renderers: single symbol, categorized, graduated and rule-based. There is no continuous color renderer, because it
is in fact only a special case of the graduated renderer. The categorized and graduated renderers can be created by
specifying a symbol and a color ramp - they will set the colors for symbols appropriately. For point layers, there
is a point displacement renderer available. For each data type (points, lines and polygons), vector symbol layer
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Figure 12.8: cpt-city dialog with hundreds of color ramps
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Figure 12.9: Vector Layer Properties Dialog
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types are available. Depending on the chosen renderer, the Style menu provides different additional sections. On
the bottom right of the symbology dialog, there is a [Symbol] button, which gives access to the Style Manager
(see Presentation). The Style Manager allows you to edit and remove existing symbols and add new ones.
After having made any needed changes, the symbol can be added to the list of current style symbols (using
[Symbol] Save in symbol library), and then it can easily be used in the future. Furthermore, you can use
the [Save Style] button to save the symbol as a QGIS layer style file (.qml) or SLD file (.sld). SLDs can be
exported from any type of renderer – single symbol, categorized, graduated or rule-based – but when importing
an SLD, either a single symbol or rule-based renderer is created. That means that categorized or graduated styles
are converted to rule-based. If you want to preserve those renderers, you have to stick to the QML format. On the
other hand, it can be very handy sometimes to have this easy way of converting styles to rule-based.
If you change the renderer type when setting the style of a vector layer the settings you made for the symbol will
be maintained. Be aware that this procedure only works for one change. If you repeat changing the renderer type
the settings for the symbol will get lost.
If the datasource of the layer is a database (PostGIS or Spatialite for example), you can save your layer style inside
a table of the database. Just click on Save Style comboxbox and choose Save in database item then fill in the
dialog to define a style name, add a description, an ui file and if the style is a default style. When loading a layer
from the database, if a style already exists for this layer, QGIS will load the layer and its style. You can add several
style in the database. Only one will be the default style anyway.
Figure 12.10: Save Style in database Dialog
Truco: Select and change multiple symbols
The Symbology allows you to select multiple symbols and right click to change color, transparency, size, or width
of selected entries.
Single Symbol Renderer
The Single Symbol Renderer is used to render all features of the layer using a single user-defined symbol. The
properties, which can be adjusted in the Style menu, depend partially on the type of layer, but all types share the
following dialog structure. In the top-left part of the menu, there is a preview of the current symbol to be rendered.
On the right part of the menu, there is a list of symbols already defined for the current style, prepared to be used
by selecting them from the list. The current symbol can be modified using the menu on the right side. If you
click on the first level in the Symbol layers dialog on the left side, it’s possible to define basic parameters like Size,
Transparency,color and Rotation. Here, the layers are joined together.
In any spinbox in this dialog you can enter expressions. E.g. you can calculate simple math like multiplying the
existing size of a point by 3 without resorting to a calculator.
If you click on the second level in the Symbol layers dialog a ‘Data-defined override’ for nearly all settings is
possible. When using a data-defined color one may want to link the color to a field ‘budged’. Here a comment
functionality is inserted.
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Figure 12.11: Single symbol line properties
Figure 12.12: Expression in Size spinbox
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/*This expression will return a color code depending on the field value.
*Negative value: red
*0 value: yellow
*Positive value: green
*/
CASE
WHEN value < 0 THEN ’#DC143C’ -- Negative value: red
WHEN value = 0 THEN ’#CCCC00’ -- Value 0: yellow
ELSE ’#228B22’ -- Positive value: green
END
static/user_manual/working_with_vector/symbol_data_defined_edit.png
Figure 12.13: Data-defined symbol with Edit... menu
Categorized Renderer
The Categorized Renderer is used to render all features from a layer, using a single user-defined symbol whose
color reflects the value of a selected feature’s attribute. The Style menu allows you to select:
The attribute (using the Column listbox or the Set column expression function, see Expressions)
The symbol (using the Symbol dialog)
The colors (using the color Ramp listbox)
Then click on Classify button to create classes from the distinct value of the attribute column. Each classes can
be disabled unchecking the checkbox at the left of the class name.
You can change symbol, value and/or label of the class, just double click on the item you want to change.
Right-click shows a contextual menu to Copy/Paste,Change color,Change transparency,Change output unit,
Change symbol width.
The [Advanced] button in the lower-right corner of the dialog allows you to set the fields containing rotation and
size scale information. For convenience, the center of the menu lists the values of all currently selected attributes
together, including the symbols that will be rendered.
The example in figure_symbology_6 shows the category rendering dialog used for the rivers layer of the QGIS
sample dataset.
Graduated Renderer
The Graduated Renderer is used to render all the features from a layer, using a single user-defined symbol whose
color reflects the assignment of a selected feature’s attribute to a class.
Like the Categorized Renderer, the Graduated Renderer allows you to define rotation and size scale from specified
columns.
Also, analogous to the Categorized Renderer, the Style tab allows you to select:
The attribute (using the Column listbox or the Set column expression function, see Expressions chapter)
The symbol (using the Symbol Properties button)
The colors (using the color Ramp list)
Additionally, you can specify the number of classes and also the mode for classifying features within the classes
(using the Mode list). The available modes are:
Equal Interval: each class has the same size (e.g. values from 0 to 16 and 4 classes, each class has a size of
4);
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Figure 12.14: Categorized Symbolizing options
Figure 12.15: Graduated Symbolizing options
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Quantile: each class will have the same number of element inside (the idea of a boxplot);
Natural Breaks (Jenks): the variance within each class is minimal while the variance between classes is
maximal;
Standard Deviation: classes are built depending on the standard deviation of the values;
Pretty Breaks: the same of natural breaks but the extremes number of each class are integers.
The listbox in the center part of the Style menu lists the classes together with their ranges, labels and symbols that
will be rendered.
Click on Classify button to create classes using the choosen mode. Each classes can be disabled unchecking the
checkbox at the left of the class name.
You can change symbol, value and/or label of the clic, just double clicking on the item you want to change.
Right-click shows a contextual menu to Copy/Paste,Change color,Change transparency,Change output unit,
Change symbol width.
The example in figure_symbology_7 shows the graduated rendering dialog for the rivers layer of the QGIS sample
dataset.
Truco: Thematic maps using an expression
Categorized and graduated thematic maps can now be created using the result of an expression. In the properties
dialog for vector layers, the attribute chooser has been augmented with a Set column expression function. So
now you no longer need to write the classification attribute to a new column in your attribute table if you want the
classification attribute to be a composite of multiple fields, or a formula of some sort.
Rule-based rendering
The Rule-based Renderer is used to render all the features from a layer, using rule based symbols whose color
reflects the assignment of a selected feature’s attribute to a class. The rules are based on SQL statements. The
dialog allows rule grouping by filter or scale, and you can decide if you want to enable symbol levels or use only
the first-matched rule.
The example in figure_symbology_8 shows the rule-based rendering dialog for the rivers layer of the QGIS sample
dataset.
To create a rule, activate an existing row by double-clicking on it, or click on ‘+’ and click on the new rule. In the
Rule properties dialog, you can define a label for the rule. Press the button to open the expression string
builder. In the Function List, click on Fields and Values to view all attributes of the attribute table to be searched.
To add an attribute to the field calculator Expression field, double click its name in the Fields and Values list.
Generally, you can use the various fields, values and functions to construct the calculation expression, or you can
just type it into the box (see Expressions). You can create a new rule by copying and pasting an existing rule with
the right mouse button. You can also use the ‘ELSE’ rule that will be run if none of the other rules on that level
match. Since QGIS 2.8 the rules appear in a tree hierarchy in the map legend. Just double-klick the rules in the
map legend and the Style menu of the layer properties appears showing the rule that is the background for the
symbol in the tree.
Point displacement
The Point Displacement Renderer works to visualize all features of a point layer, even if they have the same
location. To do this, the symbols of the points are placed on a displacement circle around a center symbol.
Truco: Export vector symbology
You have the option to export vector symbology from QGIS into Google *.kml, *.dxf and MapInfo *.tab files. Just
open the right mouse menu of the layer and click on Save selection as to specify the name of the output file and
its format. In the dialog, use the Symbology export menu to save the symbology either as Feature symbology
or as Symbol layer symbology . If you have used symbol layers, it is recommended to use the second setting.
Inverted Polygon
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Figure 12.16: Rule-based Symbolizing options
Figure 12.17: Point displacement dialog
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Inverted polygon renderer allows user to define a symbol to fill in outside of the layer’s polygons. As before you
can select subrenderers. These subrenderers are the same as for the main renderers.
Figure 12.18: Inverted Polygon dialog
Truco: Switch quickly between styles
Once you created one of the above mentioned styles you can right-klick on the layer and choose Styles Add to
save your style. Now you can easily switch between styles you created using the Styles menu again.
Heatmap
With the Heatmap renderer you can create live dynamic heatmaps for (multi)point layers. You can specify the
heatmap radius in pixels, mm or map units, choose a color ramp for the heatmap style and use a slider for selecting
a tradeoff between render speed and quality. When adding or removing a feature the heatmap renderer updates the
heatmap style automatically.
Color Picker
Regardless the type of style to be used, the select color dialog will show when you click to choose a color - either
border or fill color. This dialog has four different tabs which allow you to select colors by color ramp,
color wheel,color swatches or color picker.
Whatever method you use, the selected color is always described through color sliders for HSV (Hue, Saturation,
Value) and RGB (Red, Green, Blue) values. There is also an opacity slider to set transparency level. On the lower
left part of the dialog you can see a comparison between the current and the new color you are presently selecting
and on the lower right part you have the option to add the color you just tweaked into a color slot button.
With color ramp or with color wheel, you can browse to all possible color combinations. There are other
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Figure 12.19: Color picker ramp tab
possibilities though. By using color swatches you can choose from a preselected list. This selected list is
populated with one of three methods: Recent colors,Standard colors or Project colors
Figure 12.20: Color picker swatcher tab
Another option is to use the color picker which allows you to sample a color from under your mouse pointer at
any part of QGIS or even from another application by pressing the space bar. Please note that the color picker is
OS dependent and is currently not supported by OSX.
Truco: quick color picker + copy/paste colors
You can quickly choose from Recent colors, from Standard colors or simply copy or paste a color by clicking the
drop-down arrow that follows a current color box.
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Figure 12.21: Quick color picker menu
Layer rendering
Layer transparency : You can make the underlying layer in the map canvas visible with
this tool. Use the slider to adapt the visibility of your vector layer to your needs. You can also make a
precise definition of the percentage of visibility in the the menu beside the slider.
Layer blending mode and Feature blending mode: You can achieve special rendering effects with these tools
that you may previously only know from graphics programs. The pixels of your overlaying and underlaying
layers are mixed through the settings described below.
Normal: This is the standard blend mode, which uses the alpha channel of the top pixel to blend with
the pixel beneath it. The colors aren’t mixed.
Lighten: This selects the maximum of each component from the foreground and background pixels.
Be aware that the results tend to be jagged and harsh.
Screen: Light pixels from the source are painted over the destination, while dark pixels are not. This
mode is most useful for mixing the texture of one layer with another layer (e.g., you can use a hillshade
to texture another layer).
Dodge: Dodge will brighten and saturate underlying pixels based on the lightness of the top pixel. So,
brighter top pixels cause the saturation and brightness of the underlying pixels to increase. This works
best if the top pixels aren’t too bright; otherwise the effect is too extreme.
Addition: This blend mode simply adds pixel values of one layer with the other. In case of values
above one (in the case of RGB), white is displayed. This mode is suitable for highlighting features.
Darken: This creates a resultant pixel that retains the smallest components of the foreground and
background pixels. Like lighten, the results tend to be jagged and harsh.
Multiply: Here, the numbers for each pixel of the top layer are multiplied with the corresponding
pixels for the bottom layer. The results are darker pictures.
Burn: Darker colors in the top layer cause the underlying layers to darken. Burn can be used to tweak
and colorise underlying layers.
Overlay: This mode combines the multiply and screen blending modes. In the resulting picture, light
parts become lighter and dark parts become darker.
Soft light: This is very similar to overlay, but instead of using multiply/screen it uses color burn/dodge.
This is supposed to emulate shining a soft light onto an image.
Hard light: Hard light is also very similar to the overlay mode. It’s supposed to emulate projecting a
very intense light onto an image.
Difference: Difference subtracts the top pixel from the bottom pixel, or the other way around, to
always get a positive value. Blending with black produces no change, as the difference with all colors
is zero.
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Subtract: This blend mode simply subtracts pixel values of one layer from the other. In case of negative
values, black is displayed.
12.3.2 Labels Menu
The Labels core application provides smart labeling for vector point, line and polygon layers, and it only
requires a few parameters. This new application also supports on-the-fly transformed layers. The core functions
of the application have been redesigned. In QGIS, there are a number of other features that improve the labeling.
The following menus have been created for labeling the vector layers:
• Text
• Formatting
• Buffer
• Background
• Shadow
• Placement
• Rendering
Let us see how the new menus can be used for various vector layers. Labeling point layers
Start QGIS and load a vector point layer. Activate the layer in the legend and click on the Layer Labeling Options
icon in the QGIS toolbar menu.
The first step is to activate the Label this layer with checkbox and select an attribute column to use for labeling.
Click if you want to define labels based on expressions - See labeling_with_expressions.
The following steps describe a simple labeling without using the Data defined override functions, which are
situated next to the drop-down menus.
You can define the text style in the Text menu (see Figure_labels_1 ). Use the Type case option to influence the text
rendering. You have the possibility to render the text ‘All uppercase’, ‘All lowercase’ or ‘Capitalize first letter’.
Use the blend modes to create effects known from graphics programs (see blend_modes).
In the Formatting menu, you can define a character for a line break in the labels with the ‘Wrap on character’
function. Use the Formatted numbers option to format the numbers in an attribute table. Here, decimal places
may be inserted. If you enable this option, three decimal places are initially set by default.
To create a buffer, just activate the Draw text buffer checkbox in the Buffer menu. The buffer color is variable.
Here, you can also use blend modes (see blend_modes).
If the color buffer’s fill checkbox is activated, it will interact with partially transparent text and give mixed
color transparency results. Turning off the buffer fill fixes that issue (except where the interior aspect of the buffer’s
stroke intersects with the text’s fill) and also allows you to make outlined text.
In the Background menu, you can define with Size X and Size Y the shape of your background. Use Size type to
insert an additional ‘Buffer’ into your background. The buffer size is set by default here. The background then
consists of the buffer plus the background in Size X and Size Y. You can set a Rotation where you can choose
between ‘Sync with label’, ‘Offset of label’ and ‘Fixed’. Using ‘Offset of label’ and ‘Fixed’, you can rotate the
background. Define an Offset X,Y with X and Y values, and the background will be shifted. When applying
Radius X,Y, the background gets rounded corners. Again, it is possible to mix the background with the underlying
layers in the map canvas using the Blend mode (see blend_modes).
Use the Shadow menu for a user-defined Drop shadow. The drawing of the background is very variable. Choose
between ‘Lowest label component’, ‘Text’, ‘Buffer’ and ‘Background’. The Offset angle depends on the orienta-
tion of the label. If you choose the Use global shadow checkbox, then the zero point of the angle is always
oriented to the north and doesn’t depend on the orientation of the label. You can influence the appearance of the
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shadow with the Blur radius. The higher the number, the softer the shadows. The appearance of the drop shadow
can also be altered by choosing a blend mode (see blend_modes).
Choose the Placement menu for the label placement and the labeling priority. Using the Offset from point
setting, you now have the option to use Quadrants to place your label. Additionally, you can alter the angle of
the label placement with the Rotation setting. Thus, a placement in a certain quadrant with a certain rotation is
possible. In the priority section you can define with which priority the labels are rendered. It interacts with labels
of the other vector layers in the map canvas. If there are labels from different layers in the same location then the
label with the higher priority will be displayed and the other will be left out.
In the Rendering menu, you can define label and feature options. Under Label options, you find the scale-based
visibility setting now. You can prevent QGIS from rendering only selected labels with the Show all labels for
this layer (including colliding labels) checkbox. Under Feature options, you can define whether every part of a
multipart feature is to be labeled. It’s possible to define whether the number of features to be labeled is limited
and to Discourage labels from covering features.
Figure 12.22: Smart labeling of vector point layers
Labeling line layers
The first step is to activate the Label this layer checkbox in the Label settings tab and select an attribute column
to use for labeling. Click if you want to define labels based on expressions - See labeling_with_expressions.
After that, you can define the text style in the Text menu. Here, you can use the same settings as for point layers.
Also, in the Formatting menu, the same settings as for point layers are possible.
The Buffer menu has the same functions as described in section labeling_point_layers.
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The Background menu has the same entries as described in section labeling_point_layers.
Also, the Shadow menu has the same entries as described in section labeling_point_layers.
In the Placement menu, you find special settings for line layers. The label can be placed Parallel,Curved
or Horizontal. With the Parallel and Curved option, you can define the position Above line,
On line and Below line. It’s possible to select several options at once. In that case, QGIS will look for the
optimal position of the label. Remember that here you can also use the line orientation for the position of the label.
Additionally, you can define a Maximum angle between curved characters when selecting the Curved option
(see Figure_labels_2 ).
You can set up a minimum distance for repeating labels. Distance can be in mm or in map units.
Some Placement setup will display more options, for example, Curved and Parallel Placements will allow the user
to set up the position of the label (above, below or on the line), distance from the line and for Curved, the user can
also setup inside/outside max angle between curved label. As for point vector layers you have the possibility to
define a Priority for the labels.
The Rendering menu has nearly the same entries as for point layers. In the Feature options, you can now Suppress
labeling of features smaller than.
Figure 12.23: Smart labeling of vector line layers
Labeling polygon layers
The first step is to activate the Label this layer checkbox and select an attribute column to use for labeling.
Click if you want to define labels based on expressions - See labeling_with_expressions.
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In the Text menu, define the text style. The entries are the same as for point and line layers.
The Formatting menu allows you to format multiple lines, also similar to the cases of point and line layers.
As with point and line layers, you can create a text buffer in the Buffer menu.
Use the Background menu to create a complex user-defined background for the polygon layer. You can use the
menu also as with the point and line layers.
The entries in the Shadow menu are the same as for point and line layers.
In the Placement menu, you find special settings for polygon layers (see Figure_labels_3). Offset from centroid,
Horizontal (slow),Around centroid,Free and Using perimeter are possible.
In the Offset from centroid settings, you can specify if the centroid is of the visible polygon or whole
polygon. That means that either the centroid is used for the polygon you can see on the map or the centroid is
determined for the whole polygon, no matter if you can see the whole feature on the map. You can place your
label with the quadrants here, and define offset and rotation. The Around centroid setting makes it possible to
place the label around the centroid with a certain distance. Again, you can define visible polygon or whole
polygon for the centroid. With the Using perimeter settings, you can define a position and a distance for the
label. For the position, Above line,On line,Below line and Line orientation dependent position
are possible.
Related to the choice of Label Placement, several options will appear. As for Point Placement you can choose the
distance for the polygon outline, repeat the label around the polygon perimeter.
As for point and line vector layers you have the possibility to define a Priority for the polygon vector layer.
The entries in the Rendering menu are the same as for line layers. You can also use Suppress labeling of features
smaller than in the Feature options.Define labels based on expressions
Figure 12.24: Smart labeling of vector polygon layers
QGIS allows to use expressions to label features. Just click the icon in the Labels menu of the properties
dialog. In figure_labels_4 you see a sample expression to label the alaska regions with name and area size, based
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on the field ‘NAME_2’, some descriptive text and the function ‘$area()’ in combination with ‘format_number()’
to make it look nicer.
Figure 12.25: Using expressions for labeling
Expression based labeling is easy to work with. All you have to take care of is, that you need to combine all
elements (strings, fields and functions) with a string concatenation sign ‘||’ and that fields a written in “double
quotes” and strings in ‘single quotes’. Let’s have a look at some examples:
# label based on two fields ’name’ and ’place’ with a comma as separater
"name" || ’, ’ || "place"
-> John Smith, Paris
# label based on two fields ’name’ and ’place’ separated by comma
’My name is ’ || "name" || ’and I live in ’ || "place"
-> My name is John Smith and I live in Paris
# label based on two fields ’name’ and ’place’ with a descriptive text
# and a line break (\n)
’My name is ’ || "name" || ’\nI live in ’ || "place"
-> My name is John Smith
I live in Paris
# create a multi-line label based on a field and the $area function
# to show the place name and its area size based on unit meter.
’The area of ’ || "place" || ’has a size of ’ || $area || ’m²’
-> The area of Paris has a size of 105000000 m²
# create a CASE ELSE condition. If the population value in field
# population is <= 50000 it is a town, otherwise a city.
’This place is a ’ || CASE WHEN "population <= 50000" THEN ’town’ ELSE ’city’ END
-> This place is a town
As you can see in the expression builder, you have hundreds of functions available to create simple and very
complex expressions to label your data in QGIS. See Expressions chapter for more information and examples on
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expressions.
Using data-defined override for labeling
With the data-defined override functions, the settings for the labeling are overridden by entries in the attribute
table. You can activate and deactivate the function with the right-mouse button. Hover over the symbol and you
see the information about the data-defined override, including the current definition field. We now describe an
example using the data-defined override function for the Move label function (see figure_labels_5 ).
1. Import lakes.shp from the QGIS sample dataset.
2. Double-click the layer to open the Layer Properties. Click on Labels and Placement. Select Offset from
centroid.
3. Look for the Data defined entries. Click the icon to define the field type for the Coordinate. Choose
‘xlabel’ for X and ‘ylabel’ for Y. The icons are now highlighted in yellow.
4. Zoom into a lake.
5. Go to the Label toolbar and click the icon. Now you can shift the label manually to another position
(see figure_labels_6 ). The new position of the label is saved in the ‘xlabel’ and ‘ylabel’ columns of the
attribute table.
Figure 12.26: Labeling of vector polygon layers with data-defined override
12.3.3 Fields Menu
Within the Fields menu, the field attributes of the selected dataset can be manipulated. The buttons
New Column and Delete Column can be used when the dataset is in Editing mode.
Edit Widget
Within the Fields menu, you also find an edit widget column. This column can be used to define values or a range
of values that are allowed to be added to the specific attribute table column. If you click on the [edit widget]
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Figure 12.27: Move labels
Figure 12.28: Dialog to select an edit widget for an attribute column
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button, a dialog opens, where you can define different widgets. These widgets are:
Checkbox: Displays a checkbox, and you can define what attribute is added to the column when the check-
box is activated or not.
Classification: Displays a combo box with the values used for classification, if you have chosen ‘unique
value’ as legend type in the Style menu of the properties dialog.
Color: Displays a color button allowing user to choose a color from the color dialog window.
Date/Time: Displays a line field which can open a calendar widget to enter a date, a time or both. Column
type must be text. You can select a custom format, pop-up a calendar, etc.
Enumeration: Opens a combo box with values that can be used within the columns type. This is currently
only supported by the PostgreSQL provider.
File name: Simplifies the selection by adding a file chooser dialog.
Hidden: A hidden attribute column is invisible. The user is not able to see its contents.
Photo: Field contains a filename for a picture. The width and height of the field can be defined.
Range: Allows you to set numeric values from a specific range. The edit widget can be either a slider or a
spin box.
Relation Reference: This widged lets you embed the feature form of the referenced layer on the feature
form of the actual layer. See Creating one to many relations.
Text edit (default): This opens a text edit field that allows simple text or multiple lines to be used. If you
choose multiple lines you can also choose html content.
Unique values: You can select one of the values already used in the attribute table. If ‘Editable’ is activated,
a line edit is shown with autocompletion support, otherwise a combo box is used.
UUID Generator: Generates a read-only UUID (Universally Unique Identifiers) field, if empty.
Value map: A combo box with predefined items. The value is stored in the attribute, the description is
shown in the combo box. You can define values manually or load them from a layer or a CSV file.
Value Relation: Offers values from a related table in a combobox. You can select layer, key column and
value column.
Webview: Field contains a URL. The width and height of the field is variable.
Nota: QGIS has an advanced ‘hidden’ option to define your own field widget using python and add it to this
impressive list of widgets. It is tricky but it is very well explained in following excellent blog that explains how
to create a real time validation widget that can be used like described widgets. See http://blog.vitu.ch/10142013-
1847/write-your-own-qgis-form-elements
With the Attribute editor layout, you can now define built-in forms (see figure_fields_2). This is usefull for data
entry jobs or to identify objects using the option auto open form when you have objects with many attributes. You
can create an editor with several tabs and named groups to present the attribute fields.
Choose ‘Drag and drop designer’ and an attribute column. Use the icon to create a category to insert a tab or
a named group (see figure_fields_3). When creating a new category, QGIS will insert a new tab or named group
for the category in the built-in form. The next step will be to assign the relevant fields to a selected category with
the icon. You can create more categories and use the same fields again.
Other options in the dialog are ‘Autogenerate’ and ‘Provide ui-file’.
Autogenerate’ just creates editors for all fields and tabulates them.
The ‘Provide ui-file’ option allows you to use complex dialogs made with the Qt-Designer. Us-
ing a UI-file allows a great deal of freedom in creating a dialog. For detailed information, see
http://nathanw.net/2011/09/05/qgis-tips-custom-feature-forms-with-python-logic/.
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QGIS dialogs can have a Python function that is called when the dialog is opened. Use this function to add extra
logic to your dialogs. An example is (in module MyForms.py):
def open(dialog,layer,feature):
geom = feature.geometry()
control = dialog.findChild(QWidged,"My line edit")
Reference in Python Init Function like so: MyForms.open
MyForms.py must live on PYTHONPATH, in .qgis2/python, or inside the project folder.
Figure 12.29: Dialog to create categories with the Attribute editor layout
12.3.4 General Menu
Use this menu to make general settings for the vector layer. There are several options available:
Layer Info
Change the display name of the layer in displayed as
Define the Layer source of the vector layer
Define the Data source encoding to define provider-specific options and to be able to read the file
Coordinate Reference System
Specify the coordinate reference system. Here, you can view or change the projection of the specific vector
layer.
Create a Spatial Index (only for OGR-supported formats)
Update Extents information for a layer
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Figure 12.30: Resulting built-in form with tabs and named groups
View or change the projection of the specific vector layer, clicking on Specify ...
Scale dependent visibility
You can set the Maximum (inclusive) and Minimum (exclusive) scale. The scale can also be set by the
[Current] buttons.
Feature subset
With the [Query Builder] button, you can create a subset of the features in the layer that will be visualized
(also refer to section Constructor de consultas).
12.3.5 Rendering Menu
QGIS 2.2 introduces support for on-the-fly feature generalisation. This can improve rendering times when drawing
many complex features at small scales. This feature can be enabled or disabled in the layer settings using the
Simplify geometry option. There is also a new global setting that enables generalisation by default for newly added
layers (see section Opciones). Note: Feature generalisation may introduce artefacts into your rendered output
in some cases. These may include slivers between polygons and inaccurate rendering when using offset-based
symbol layers.
12.3.6 Display Menu
This menu is specifically created for Map Tips. It includes a new feature: Map Tip display text in HTML.
While you can still choose a Field to be displayed when hovering over a feature on the map, it is now possible
to insert HTML code that creates a complex display when hovering over a feature. To activate Map Tips, select
the menu option View MapTips. Figure Display 1 shows an example of HTML code.
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Figure 12.31: General menu in vector layers properties dialog
Figure 12.32: HTML code for map tip
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Figure 12.33: Map tip made with HTML code
12.3.7 Actions Menu
QGIS provides the ability to perform an action based on the attributes of a feature. This can be used to
perform any number of actions, for example, running a program with arguments built from the attributes of a
feature or passing parameters to a web reporting tool.
Actions are useful when you frequently want to run an external application or view a web page based on one or
more values in your vector layer. They are divided into six types and can be used like this:
Generic, Mac, Windows and Unix actions start an external process.
Python actions execute a Python expression.
Generic and Python actions are visible everywhere.
Mac, Windows and Unix actions are visible only on the respective platform (i.e., you can define three ‘Edit’
actions to open an editor and the users can only see and execute the one ‘Edit’ action for their platform to
run the editor).
There are several examples included in the dialog. You can load them by clicking on [Add default actions]. One
example is performing a search based on an attribute value. This concept is used in the following discussion.
Defining Actions
Attribute actions are defined from the vector Layer Properties dialog. To define an action, open the vector Layer
Properties dialog and click on the Actions menu. Go to the Action properties. Select ‘Generic’ as type and provide
a descriptive name for the action. The action itself must contain the name of the application that will be executed
when the action is invoked. You can add one or more attribute field values as arguments to the application. When
the action is invoked, any set of characters that start with a %followed by the name of a field will be replaced by
the value of that field. The special characters %% will be replaced by the value of the field that was selected from
the identify results or attribute table (see using_actions below). Double quote marks can be used to group text into
a single argument to the program, script or command. Double quotes will be ignored if preceded by a backslash.
If you have field names that are substrings of other field names (e.g., col1 and col10), you should indicate that
by surrounding the field name (and the % character) with square brackets (e.g., [%col10]). This will prevent
the %col10 field name from being mistaken for the %col1 field name with a 0on the end. The brackets will be
removed by QGIS when it substitutes in the value of the field. If you want the substituted field to be surrounded
by square brackets, use a second set like this: [[%col10]].
Using the Identify Features tool, you can open the Identify Results dialog. It includes a (Derived) item that contains
information relevant to the layer type. The values in this item can be accessed in a similar way to the other fields
by preceeding the derived field name with (Derived).. For example, a point layer has an Xand Yfield, and
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Figure 12.34: Overview action dialog with some sample actions
the values of these fields can be used in the action with %(Derived).X and %(Derived).Y. The derived
attributes are only available from the Identify Results dialog box, not the Attribute Table dialog box.
Two example actions are shown below:
konqueror http://www.google.com/search?q=%nam
konqueror http://www.google.com/search?q=%%
In the first example, the web browser konqueror is invoked and passed a URL to open. The URL performs a
Google search on the value of the nam field from our vector layer. Note that the application or script called
by the action must be in the path, or you must provide the full path. To be certain, we could rewrite the first
example as: /opt/kde3/bin/konqueror http://www.google.com/search?q=%nam. This will
ensure that the konqueror application will be executed when the action is invoked.
The second example uses the %% notation, which does not rely on a particular field for its value. When the action
is invoked, the %% will be replaced by the value of the selected field in the identify results or attribute table.
Using Actions
Actions can be invoked from either the Identify Results dialog, an Attribute Table dialog or from Run Fea-
ture Action (recall that these dialogs can be opened by clicking Identify Features or Open Attribute Table or
Run Feature Action). To invoke an action, right click on the record and choose the action from the pop-up menu. Ac-
tions are listed in the popup menu by the name you assigned when defining the action. Click on the action you
wish to invoke.
If you are invoking an action that uses the %% notation, right-click on the field value in the Identify Results dialog
or the Attribute Table dialog that you wish to pass to the application or script.
Here is another example that pulls data out of a vector layer and inserts it into a file using bash and the echo com-
mand (so it will only work on or perhaps ). The layer in question has fields for a species name taxon_name,
latitude lat and longitude long. We would like to be able to make a spatial selection of localities and export
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these field values to a text file for the selected record (shown in yellow in the QGIS map area). Here is the action
to achieve this:
bash -c "echo \"%taxon_name %lat %long\" >> /tmp/species_localities.txt"
After selecting a few localities and running the action on each one, opening the output file will show something
like this:
Acacia mearnsii -34.0800000000 150.0800000000
Acacia mearnsii -34.9000000000 150.1200000000
Acacia mearnsii -35.2200000000 149.9300000000
Acacia mearnsii -32.2700000000 150.4100000000
As an exercise, we can create an action that does a Google search on the lakes layer. First, we need to determine
the URL required to perform a search on a keyword. This is easily done by just going to Google and doing a
simple search, then grabbing the URL from the address bar in your browser. From this little effort, we see that the
format is http://google.com/search?q=qgis, where QGIS is the search term. Armed with this information, we can
proceed:
1. Make sure the lakes layer is loaded.
2. Open the Layer Properties dialog by double-clicking on the layer in the legend, or right-click and choose
Properties from the pop-up menu.
3. Click on the Actions menu.
4. Enter a name for the action, for example Google Search.
5. For the action, we need to provide the name of the external program to run. In this case, we can use Firefox.
If the program is not in your path, you need to provide the full path.
6. Following the name of the external application, add the URL used for doing a Google search, up to but not
including the search term: http://google.com/search?q=
7. The text in the Action field should now look like this: firefox http://google.com/search?q=
8. Click on the drop-down box containing the field names for the lakes layer. It’s located just to the left of
the [Insert Field] button.
9. From the drop-down box, select ‘NAMES’ and click [Insert Field].
10. Your action text now looks like this:
firefox http://google.com/search?q=%NAMES
11. To finalize the action, click the [Add to action list] button.
This completes the action, and it is ready to use. The final text of the action should look like this:
firefox http://google.com/search?q=%NAMES
We can now use the action. Close the Layer Properties dialog and zoom in to an area of interest. Make sure the
lakes layer is active and identify a lake. In the result box you’ll now see that our action is visible:
When we click on the action, it brings up Firefox and navigates to the URL
http://www.google.com/search?q=Tustumena. It is also possible to add further attribute fields to the ac-
tion. Therefore, you can add a +to the end of the action text, select another field and click on [Insert Field]. In
this example, there is just no other field available that would make sense to search for.
You can define multiple actions for a layer, and each will show up in the Identify Results dialog.
There are all kinds of uses for actions. For example, if you have a point layer containing locations of images or
photos along with a file name, you could create an action to launch a viewer to display the image. You could also
use actions to launch web-based reports for an attribute field or combination of fields, specifying them in the same
way we did in our Google search example.
We can also make more complex examples, for instance, using Python actions.
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Figure 12.35: Select feature and choose action
Usually, when we create an action to open a file with an external application, we can use absolute paths, or
eventually relative paths. In the second case, the path is relative to the location of the external program executable
file. But what about if we need to use relative paths, relative to the selected layer (a file-based one, like a shapefile
or SpatiaLite)? The following code will do the trick:
command = "firefox";
imagerelpath = "images_test/test_image.jpg";
layer = qgis.utils.iface.activeLayer();
import os.path;
layerpath = layer.source() if layer.providerType() == ’ogr’
else (qgis.core.QgsDataSourceURI(layer.source()).database()
if layer.providerType() == ’spatialite’ else None);
path = os.path.dirname(str(layerpath));
image = os.path.join(path,imagerelpath);
import subprocess;
subprocess.Popen( [command, image ] );
We just have to remember that the action is one of type Python and the command and imagerelpath variables must
be changed to fit our needs.
But what about if the relative path needs to be relative to the (saved) project file? The code of the Python action
would be:
command="firefox";
imagerelpath="images/test_image.jpg";
projectpath=qgis.core.QgsProject.instance().fileName();
import os.path; path=os.path.dirname(str(projectpath)) if projectpath != ’’ else None;
image=os.path.join(path, imagerelpath);
import subprocess;
subprocess.Popen( [command, image ] );
Another Python action example is the one that allows us to add new layers to the project. For instance, the
following examples will add to the project respectively a vector and a raster. The names of the files to be added to
the project and the names to be given to the layers are data driven (filename and layername are column names of
the table of attributes of the vector where the action was created):
qgis.utils.iface.addVectorLayer(’/yourpath/[% "filename" %].shp’,’[% "layername" %]’,
’ogr’)
To add a raster (a TIF image in this example), it becomes:
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qgis.utils.iface.addRasterLayer(’/yourpath/[% "filename" %].tif’,’[% "layername" %]
’)
12.3.8 Joins Menu
The Joins menu allows you to join a loaded attribute table to a loaded vector layer. After clicking , the
Add vector join dialog appears. As key columns, you have to define a join layer you want to connect with the
target vector layer. Then, you have to specify the join field that is common to both the join layer and the target
layer. Now you can also specify a subset of fields from the joined layer based on the checkbox Choose which
fields are joined. As a result of the join, all information from the join layer and the target layer are displayed in
the attribute table of the target layer as joined information. If you specified a subset of fields only these fields are
displayed in the attribute table of the target layer.
QGIS currently has support for joining non-spatial table formats supported by OGR (e.g., CSV, DBF and Excel),
delimited text and the PostgreSQL provider (see figure_joins_1).
Figure 12.36: Join an attribute table to an existing vector layer
Additionally, the add vector join dialog allows you to:
Cache join layer in virtual memory
Create attribute index on the join field
Choose which fields are joined
Create a Custom field name prefix
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12.3.9 Diagrams Menu
The Diagrams menu allows you to add a graphic overlay to a vector layer (see figure_diagrams_1).
The current core implementation of diagrams provides support for pie charts, text diagrams and histograms.
The menu is divided into four tabs: Appearance,Size,Postion and Options.
In the cases of the text diagram and pie chart, text values of different data columns are displayed one below the
other with a circle or a box and dividers. In the Size tab, diagram size is based on a fixed size or on linear scaling
according to a classification attribute. The placement of the diagrams, which is done in the Position tab, interacts
with the new labeling, so position conflicts between diagrams and labels are detected and solved. In addition,
chart positions can be fixed manually.
Figure 12.37: Vector properties dialog with diagram menu
We will demonstrate an example and overlay on the Alaska boundary layer a text diagram showing temperature
data from a climate vector layer. Both vector layers are part of the QGIS sample dataset (see section Datos de
ejemplo).
1. First, click on the Load Vector icon, browse to the QGIS sample dataset folder, and load the two vector
shape layers alaska.shp and climate.shp.
2. Double click the climate layer in the map legend to open the Layer Properties dialog.
3. Click on the Diagrams menu, activate Display diagrams, and from the Diagram type combo box,
select ‘Text diagram’.
4. In the Appearance tab, we choose a light blue as background color, and in the Size tab, we set a fixed size
to 18 mm.
5. In the Position tab, placement could be set to ‘Around Point’.
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6. In the diagram, we want to display the values of the three columns T_F_JAN,T_F_JUL and T_F_MEAN.
First select T_F_JAN as Attributes and click the button, then T_F_JUL, and finally T_F_MEAN.
7. Now click [Apply] to display the diagram in the QGIS main window.
8. You can adapt the chart size in the Size tab. Deactivate the Fixed size and set the size of the diagrams on
the basis of an attribute with the [Find maximum value] button and the Size menu. If the diagrams appear
too small on the screen, you can activate the Increase size of small diagrams checkbox and define the
minimum size of the diagrams.
9. Change the attribute colors by double clicking on the color values in the Assigned attributes field. Fig-
ure_diagrams_2 gives an idea of the result.
10. Finally, click [Ok].
Figure 12.38: Diagram from temperature data overlayed on a map
Remember that in the Position tab, a Data defined position of the diagrams is possible. Here, you can use
attributes to define the position of the diagram. You can also set a scale-dependent visibility in the Appearance
tab.
The size and the attributes can also be an expression. Use the button to add an expression. See Expressions
chapter for more information and example.
12.3.10 Metadata Menu
The Metadata menu consists of Description,Attribution,MetadataURL and Properties sections.
In the Properties section, you get general information about the layer, including specifics about the type and
location, number of features, feature type, and editing capabilities. The Extents table provides you with layer
extent information and the Layer Spatial Reference System, which is information about the CRS of the layer. This
is a quick way to get information about the layer.
Additionally, you can add or edit a title and abstract for the layer in the Description section. It’s also possible to
define a Keyword list here. These keyword lists can be used in a metadata catalogue. If you want to use a title from
an XML metadata file, you have to fill in a link in the DataUrl field. Use Attribution to get attribute data from an
XML metadata catalogue. In MetadataUrl, you can define the general path to the XML metadata catalogue. This
information will be saved in the QGIS project file for subsequent sessions and will be used for QGIS server.
.
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Figure 12.39: Metadata menu in vector layers properties dialog
12.4 Expressions
The Expressions feature are available through the field calculator or the add a new column button in the attribut
table or the Field tab in the Layer properties ; through the graduaded, categorized and rule-based rendering in the
Style tab of the Layer properties ; through the expression-based labeling in the Labeling core application
; through the feature selection and through the diagram tab of the Layer properties as well as the Main properties
of the label item and the Atlas generation tab in the Print Composer.
They are a powerful way to manipulate attribute value in order to dynamically change the final value in order to
change the geometry style, the content of the label, the value for diagram, select some feature or create virtual
column.
12.4.1 Functions List
The Function List contains functions as well as fields and values. View the help function in the Selected Func-
tion Help. In Expression you see the calculation expressions you create with the Function List. For the most
commonly used operators, see Operators.
In the Function List, click on Fields and Values to view all attributes of the attribute table to be searched. To add
an attribute to the Field calculator Expression field, double click its name in the Fields and Values list. Generally,
you can use the various fields, values and functions to construct the calculation expression, or you can just type it
into the box. To display the values of a field, you just right click on the appropriate field. You can choose between
Load top 10 unique values and Load all unique values. On the right side, the Field Values list opens with the
unique values. To add a value to the Field calculator Expression box, double click its name in the Field Values
list.
The Operators,Math,Conversions,String,Geometry and Record groups provide several functions. In Operators,
you find mathematical operators. Look in Math for mathematical functions. The Conversions group contains
functions that convert one data type to another. The String group provides functions for data strings. In the
Geometry group, you find functions for geometry objects. With Record group functions, you can add a numeration
to your data set. To add a function to the Field calculator Expression box, click on the > and then double click the
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function.
Operators
This group contains operators (e.g., +, -, *).
a + b a plus b
a - b a minus b
a*b a multiplied by b
a / b a divided by b
a % b a modulo b (for example, 7 % 2 = 1, or 2 fits into 7 three
times with remainder 1)
a ^ b a power b (for example, 2^2=4 or 2^3=8)
a = b a and b are equal
a > b a is larger than b
a < b a is smaller than b
a <> b a and b are not equal
a != b a and b are not equal
a <= b a is less than or equal to b
a >= b a is larger than or equal to b
a ~ b a matches the regular expression b
+ a positive sign
- a negative value of a
|| joins two values together into a string ’Hello’ || ’ world’
LIKE returns 1 if the string matches the supplied pattern
ILIKE returns 1 if the string matches case-insensitive the supplied
pattern (ILIKE can be used instead of LIKE to make the match
case-insensitive)
IS returns 1 if a is the same as b
OR returns 1 when condition a or b is true
AND returns 1 when condition a and b are true
NOT returns 1 if a is not the same as b
column name "column name" value of the field column name, take
care to not be confused with simple
quote, see below
’string’ a string value, take care to not be
confused with double quote, see above
NULL null value
a IS NULL a has no value
a IS NOT NULL a has a value
a IN (value[,value]) a is below the values listed
a NOT IN (value[,value]) a is not below the values listed
Some examples:
Joins a string and a value from a column name:
’My feature’s id is: ’ || "gid"
Test if the “description” attribute field starts with the ‘Hello’ string in the value (note the position of the %
character):
"description" LIKE ’Hello%’
Conditionals
This group contains functions to handle conditional checks in expressions.
CASE evaluates multiple expressions and returns a
result
CASE ELSE evaluates multiple expressions and returns a
result
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coalesce returns the first non-NULL value from the
expression list
regexp_match returns true if any part of a string matches
the supplied regular expression
Some example:
Send back a value if the first condition is true, else another value:
CASE WHEN "software" LIKE ’%QGIS%’ THEN ’QGIS’ ELSE ’Other’
Mathematical Functions
This group contains math functions (e.g., square root, sin and cos).
sqrt(a) square root of a
abs returns the absolute value of a number
sin(a) sine of a
cos(a) cosine of a
tan(a) tangent of a
asin(a) arcsin of a
acos(a) arccos of a
atan(a) arctan of a
atan2(y,x) arctan of y/x using the signs of the two
arguments to determine the quadrant of the
result
exp exponential of a value
ln value of the natural logarithm of the passed
expression
log10 value of the base 10 logarithm of the passed
expression
log value of the logarithm of the passed value
and base
round round to number of decimal places
rand random integer within the range specified by
the minimum
and maximum argument (inclusive)
randf random float within the range specified by
the minimum
and maximum argument (inclusive)
max largest value in a set of values
min smallest value in a set of values
clamp restricts an input value to a specified
range
scale_linear transforms a given value from an input
domain to an output
range using linear interpolation
scale_exp transforms a given value from an input
domain to an output
range using an exponential curve
floor rounds a number downwards
ceil rounds a number upwards
$pi pi as value for calculations
Conversions
This group contains functions to convert one data type to another (e.g., string to integer, integer to string).
toint converts a string to integer number
toreal converts a string to real number
tostring converts number to string
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todatetime converts a string into Qt data time type
todate converts a string into Qt data type
totime converts a string into Qt time type
tointerval converts a string to an interval type (can be
used to take days, hours, months, etc. off a
date)
Date and Time Functions
This group contains functions for handling date and time data.
$now current date and time
age difference between two dates
year extract the year part from a date, or the number of years from
an interval
month extract the month part from a date, or the number of months
from an interval
week extract the week number from a date, or the number of weeks
from an interval
day extract the day from a date, or the number of days from an
interval
hour extract the hour from a datetime or time, or the number
of hours from an interval
minute extract the minute from a datetime or time, or the number
of minutes from an interval
second extract the second from a datetime or time, or the number
of minutes from an interval
Some example:
Get the month and the year of today in the format “10/2014”
month($now) || ’/’ || year($now)
String Functions
This group contains functions that operate on strings (e.g., that replace, convert to upper case).
lower convert string a to lower case
upper convert string a to upper case
title converts all words of a string to title
case (all words lower case with leading
capital letter)
trim removes all leading and trailing white
space (spaces, tabs, etc.) from a string
wordwrap returns a string wrapped to a maximum/
minimum number of characters
length length of string a
replace returns a string with the supplied string
replaced
regexp_replace(a,this,that) returns a string with the supplied regular
expression replaced
regexp_substr returns the portion of a string which matches
a supplied regular expression
substr(*a*,from,len) returns a part of a string
concat concatenates several strings to one
strpos returns the index of a regular expression
in a string
left returns a substring that contains the n
leftmost characters of the string
right returns a substring that contains the n
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rightmost characters of the string
rpad returns a string with supplied width padded
using the fill character
lpad returns a string with supplied width padded
using the fill character
format formats a string using supplied arguments
format_number returns a number formatted with the locale
separator for thousands (also truncates the
number to the number of supplied places)
format_date formats a date type or string into a custom
string format
Color Functions
This group contains functions for manipulating colors.
color_rgb returns a string representation of a color based on its
red, green, and blue components
color_rgba returns a string representation of a color based on its
red, green, blue, and alpha (transparency) components
ramp_color returns a string representing a color from a color ramp
color_hsl returns a string representation of a color based on its
hue, saturation, and lightness attributes
color_hsla returns a string representation of a color based on its
hue, saturation, lightness and alpha (transparency)
attributes
color_hsv returns a string representation of a color based on its
hue, saturation, and value attributes
color_hsva returns a string representation of a color based on its
hue, saturation, value and alpha (transparency) attributes
color_cmyk returns a string representation of a color based on its
cyan, magenta, yellow and black components
color_cmyka returns a string representation of a color based on its
cyan, magenta, yellow, black and alpha (transparency)
components
Geometry Functions
This group contains functions that operate on geometry objects (e.g., length, area).
$geometry returns the geometry of the current feature (can be used
for processing with other functions)
$area returns the area size of the current feature
$length returns the length size of the current feature
$perimeter returns the perimeter length of the current feature
$x returns the x coordinate of the current feature
$y returns the y coordinate of the current feature
xat retrieves the nth x coordinate of the current feature.
n given as a parameter of the function
yat retrieves the nth y coordinate of the current feature.
n given as a parameter of the function
xmin returns the minimum x coordinate of a geometry.
Calculations are in the Spatial Reference System of this
Geometry
xmax returns the maximum x coordinate of a geometry.
Calculations are in the Spatial Reference System of this
Geometry
ymin returns the minimum y coordinate of a geometry.
Calculations are in the Spatial Reference System of this
Geometry
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ymax returns the maximum y coordinate of a geometry.
Calculations are in the Spatial Reference System of this
Geometry
geomFromWKT returns a geometry created from a well-known text (WKT)
representation
geomFromGML returns a geometry from a GML representation of geometry
bbox
disjoint returns 1 if the geometries do not share any space
together
intersects returns 1 if the geometries spatially intersect
(share any portion of space) and 0 if they don’t
touches returns 1 if the geometries have at least one point in
common, but their interiors do not intersect
crosses returns 1 if the supplied geometries have some, but not
all, interior points in common
contains returns true if and only if no points of b lie in the
exterior of a, and at least one point of the interior
of b lies in the interior of a
overlaps returns 1 if the geometries share space, are of the
same dimension, but are not completely contained by
each other
within returns 1 if geometry a is completely inside geometry b
buffer returns a geometry that represents all points whose
distance from this geometry is less than or equal to
distance
centroid returns the geometric center of a geometry
bounds returns a geometry which represents the bounding box of
an input geometry. Calculations are in the Spatial
Reference System of this Geometry.
bounds_width returns the width of the bounding box of a geometry.
Calculations are in the Spatial Reference System of
this Geometry.
bounds_height returns the height of the bounding box of a geometry.
Calculations are in the Spatial Reference System of
this Geometry.
convexHull returns the convex hull of a geometry (this represents
the minimum convex geometry that encloses all geometries
within the set)
difference returns a geometry that represents that part of geometry
a that does not intersect with geometry b
distance returns the minimum distance (based on spatial ref)
between two geometries in projected units
intersection returns a geometry that represents the shared portion
of geometry a and geometry b
symDifference returns a geometry that represents the portions of a and
b that do not intersect
combine returns the combination of geometry a and geometry b
union returns a geometry that represents the point set union of
the geometries
geomToWKT returns the well-known text (WKT) representation of the
geometry without SRID metadata
geometry returns the feature’s geometry
transform returns the geometry transformed from the source CRS to
the dest CRS
Record Functions
This group contains functions that operate on record identifiers.
$rownum returns the number of the current row
$id returns the feature id of the current row
$currentfeature returns the current feature being evaluated.
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This can be used with the ’attribute’ function
to evaluate attribute values from the current
feature.
$scale returns the current scale of the map canvas
$uuid generates a Universally Unique Identifier (UUID)
for each row. Each UUID is 38 characters long.
getFeature returns the first feature of a layer matching a
given attribute value.
attribute returns the value of a specified attribute from
a feature.
$map returns the id of the current map item if the map
is being drawn in a composition, or "canvas" if
the map is being drawn within the main QGIS
window.
Fields and Values
Contains a list of fields from the layer. Sample values can also be accessed via right-click.
Select the field name from the list, then right-click to access a context menu with options to load sample values
from the selected field.
Fields name should be double-quoted. Values or string should be simple-quoted.
.
12.5 Editing
QGIS supports various capabilities for editing OGR, SpatiaLite, PostGIS, MSSQL Spatial and Oracle Spatial
vector layers and tables.
Nota: The procedure for editing GRASS layers is different - see section Digitizing and editing a GRASS vector
layer for details.
Truco: Concurrent Edits
This version of QGIS does not track if somebody else is editing a feature at the same time as you are. The last
person to save their edits wins.
12.5.1 Setting the Snapping Tolerance and Search Radius
Before we can edit vertices, we must set the snapping tolerance and search radius to a value that allows us an
optimal editing of the vector layer geometries.
Snapping tolerance
Snapping tolerance is the distance QGIS uses to search for the closest vertex and/or segment you are trying to
connect to when you set a new vertex or move an existing vertex. If you aren’t within the snapping tolerance,
QGIS will leave the vertex where you release the mouse button, instead of snapping it to an existing vertex and/or
segment. The snapping tolerance setting affects all tools that work with tolerance.
1. A general, project-wide snapping tolerance can be defined by choosing Settings Options. On Mac, go
to QGIS Preferences.... On Linux: Edit Options. In the Digitizing tab, you can select between
‘to vertex’, ‘to segment’ or ‘to vertex and segment’ as default snap mode. You can also define a default
snapping tolerance and a search radius for vertex edits. The tolerance can be set either in map units or in
pixels. The advantage of choosing pixels is that the snapping tolerance doesn’t have to be changed after
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zoom operations. In our small digitizing project (working with the Alaska dataset), we define the snapping
units in feet. Your results may vary, but something on the order of 300 ft at a scale of 1:10000 should be a
reasonable setting.
2. A layer-based snapping tolerance can be defined by choosing Settings (or File )Snapping options... to
enable and adjust snapping mode and tolerance on a layer basis (see figure_edit_1 ).
Note that this layer-based snapping overrides the global snapping option set in the Digitizing tab. So, if you need
to edit one layer and snap its vertices to another layer, then enable snapping only on the snap to layer, then
decrease the global snapping tolerance to a smaller value. Furthermore, snapping will never occur to a layer that
is not checked in the snapping options dialog, regardless of the global snapping tolerance. So be sure to mark the
checkbox for those layers that you need to snap to.
Figure 12.40: Edit snapping options on a layer basis (Advanced mode)
The Snapping options enables you to make a quick and simple general setting for all layers in the project so that
the pointer snaps to all existing vertices and/or segments when using the ‘All layers’ snapping mode. In most cases
it is sufficient to use this snapping mode.
It is important to consider that the per-layer tolerance in ‘map units’ was actually in layer units. So if working
with a layer in WGS84 reprojected to UTM, setting tolerance to 1 map unit (i.e. 1 meter) wouldn’t work correctly
because the units would be actually degrees. So now the ‘map units’ has been relabeled to ‘layer units’ and the
new entry ‘map units’ operates with units of the map view. While working with ‘on-the-fly’ CRS transformation
it is now possible to use a snapping tolerance that refers to either the units of the reprojected layer (setting ‘layer
units’) or the units of the map view (setting ‘map units’).
Search radius
Search radius is the distance QGIS uses to search for the closest vertex you are trying to move when you click
on the map. If you aren’t within the search radius, QGIS won’t find and select any vertex for editing, and it will
pop up an annoying warning to that effect. Snap tolerance and search radius are set in map units or pixels, so you
may find you need to experiment to get them set right. If you specify too big of a tolerance, QGIS may snap to the
wrong vertex, especially if you are dealing with a large number of vertices in close proximity. Set search radius
too small, and it won’t find anything to move.
The search radius for vertex edits in layer units can be defined in the Digitizing tab under Settings Options.
This is the same place where you define the general, project- wide snapping tolerance.
12.5.2 Zooming and Panning
Before editing a layer, you should zoom in to your area of interest. This avoids waiting while all the vertex markers
are rendered across the entire layer.
Apart from using the pan and zoom-in /zoom-out icons on the toolbar with the mouse, navigating can also
be done with the mouse wheel, spacebar and the arrow keys.
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Zooming and panning with the mouse wheel
While digitizing, you can press the mouse wheel to pan inside of the main window, and you can roll the mouse
wheel to zoom in and out on the map. For zooming, place the mouse cursor inside the map area and roll it forward
(away from you) to zoom in and backwards (towards you) to zoom out. The mouse cursor position will be the
center of the zoomed area of interest. You can customize the behavior of the mouse wheel zoom using the Map
tools tab under the Settings Options menu.
Panning with the arrow keys
Panning the map during digitizing is possible with the arrow keys. Place the mouse cursor inside the map area,
and click on the right arrow key to pan east, left arrow key to pan west, up arrow key to pan north, and down arrow
key to pan south.
You can also use the space bar to temporarily cause mouse movements to pan the map. The PgUp and PgDown
keys on your keyboard will cause the map display to zoom in or out without interrupting your digitizing session.
12.5.3 Topological editing
Besides layer-based snapping options, you can also define topological functionalities in the Snapping options...
dialog in the Settings (or File) menu. Here, you can define Enable topological editing, and/or for polygon
layers, you can activate the column Avoid Int., which avoids intersection of new polygons.
Enable topological editing
The option Enable topological editing is for editing and maintaining common boundaries in polygon mosaics.
QGIS ‘detects’ a shared boundary in a polygon mosaic, so you only have to move the vertex once, and QGIS will
take care of updating the other boundary.
Avoid intersections of new polygons
The second topological option in the Avoid Int. column, called Avoid intersections of new polygons, avoids
overlaps in polygon mosaics. It is for quicker digitizing of adjacent polygons. If you already have one polygon,
it is possible with this option to digitize the second one such that both intersect, and QGIS then cuts the second
polygon to the common boundary. The advantage is that you don’t have to digitize all vertices of the common
boundary.
Enable snapping on intersections
Another option is to use Enable snapping on intersection. It allows you to snap on an intersection of back-
ground layers, even if there’s no vertex on the intersection.
12.5.4 Digitizing an existing layer
By default, QGIS loads layers read-only. This is a safeguard to avoid accidentally editing a layer if there is a
slip of the mouse. However, you can choose to edit any layer as long as the data provider supports it, and the
underlying data source is writable (i.e., its files are not read-only).
In general, tools for editing vector layers are divided into a digitizing and an advanced digitiz-
ing toolbar, described in section Advanced digitizing. You can select and unselect both under View
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Toolbars . Using the basic digitizing tools, you can perform the following functions:
Icon Purpose Icon Purpose
Current edits Toggle editing
Adding Features: Capture Point Adding Features: Capture Line
Adding Features: Capture Polygon Move Feature
Node Tool Delete Selected
Cut Features Copy Features
Paste Features Save layer edits
Table Editing: Vector layer basic editing toolbar
All editing sessions start by choosing the Toggle editing option. This can be found in the context menu after right
clicking on the legend entry for a given layer.
Alternatively, you can use the Toggle Editing Toggle editing button from the digitizing toolbar to start or stop the
editing mode. Once the layer is in edit mode, markers will appear at the vertices, and additional tool buttons on
the editing toolbar will become available.
Truco: Save Regularly
Remember to Save Layer Edits regularly. This will also check that your data source can accept all the changes.
Adding Features
You can use the Add Feature,Add Feature or Add Feature icons on the toolbar to put the QGIS cursor into
digitizing mode.
For each feature, you first digitize the geometry, then enter its attributes. To digitize the geometry, left-click on
the map area to create the first point of your new feature.
For lines and polygons, keep on left-clicking for each additional point you wish to capture. When you have
finished adding points, right-click anywhere on the map area to confirm you have finished entering the geometry
of that feature.
The attribute window will appear, allowing you to enter the information for the new feature. Figure_edit_2 shows
setting attributes for a fictitious new river in Alaska. In the Digitizing menu under the Settings Options menu,
you can also activate Suppress attributes pop-up windows after each created feature and Reuse last entered
attribute values.
Figure 12.41: Enter Attribute Values Dialog after digitizing a new vector feature
With the Move Feature(s) icon on the toolbar, you can move existing features.
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Truco: Attribute Value Types
For editing, the attribute types are validated during entry. Because of this, it is not possible to enter a number into
a text column in the dialog Enter Attribute Values or vice versa. If you need to do so, you should edit the attributes
in a second step within the Attribute table dialog.
Current Edits
This feature allows the digitization of multiple layers. Choose Save for Selected Layers to save all changes
you made in multiple layers. You also have the opportunity to Rollback for Selected Layers, so that the
digitization may be withdrawn for all selected layers. If you want to stop editing the selected layers, Cancel
for Selected Layer(s) is an easy way.
The same functions are available for editing all layers of the project.
Node Tool
For shapefile-based layers as well as SpatialLite, PostgreSQL/PostGIS, MSSQL Spatial, and Oracle Spatial tables,
the Node Tool provides manipulation capabilities of feature vertices similar to CAD programs. It is possible to
simply select multiple vertices at once and to move, add or delete them altogether. The node tool also works with
‘on the fly’ projection turned on, and it supports the topological editing feature. This tool is, unlike other tools in
QGIS, persistent, so when some operation is done, selection stays active for this feature and tool. If the node tool
is unable to find any features, a warning will be displayed.
It is important to set the property Settings Options Digitizing Search Radius: to a number
greater than zero (i.e., 10). Otherwise, QGIS will not be able to tell which vertex is being edited.
Truco: Vertex Markers
The current version of QGIS supports three kinds of vertex markers: ‘Semi-transparent circle’, ‘Cross’ and ‘None’.
To change the marker style, choose Options from the Settings menu, click on the Digitizing tab and select the
appropriate entry.
Basic operations
Start by activating the Node Tool and selecting a feature by clicking on it. Red boxes will appear at each vertex
of this feature.
Selecting vertices: You can select vertices by clicking on them one at a time, by clicking on an edge to
select the vertices at both ends, or by clicking and dragging a rectangle around some vertices. When a
vertex is selected, its color changes to blue. To add more vertices to the current selection, hold down the
Ctrl key while clicking. Hold down Ctrl or Shift when clicking to toggle the selection state of vertices
(vertices that are currently unselected will be selected as usual, but also vertices that are already selected
will become unselected).
Adding vertices: To add a vertex, simply double click near an edge and a new vertex will appear on the
edge near to the cursor. Note that the vertex will appear on the edge, not at the cursor position; therefore, it
should be moved if necessary.
Deleting vertices: After selecting vertices for deletion, click the Delete key. Note that you cannot use the
Node Tool to delete a complete feature; QGIS will ensure it retains the minimum number of vertices for
the feature type you are working on. To delete a complete feature use the Delete Selected tool.
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Moving vertices: Select all the vertices you want to move. Click on a selected vertex or edge and drag in
the direction you wish to move. All the selected vertices will move together. If snapping is enabled, the
whole selection can jump to the nearest vertex or line.
Each change made with the node tool is stored as a separate entry in the Undo dialog. Remember that all operations
support topological editing when this is turned on. On-the-fly projection is also supported, and the node tool
provides tooltips to identify a vertex by hovering the pointer over it.
Cutting, Copying and Pasting Features
Selected features can be cut, copied and pasted between layers in the same QGIS project, as long as destination
layers are set to Toggle editing beforehand.
Features can also be pasted to external applications as text. That is, the features are represented in CSV format,
with the geometry data appearing in the OGC Well-Known Text (WKT) format.
However, in this version of QGIS, text features from outside QGIS cannot be pasted to a layer within QGIS. When
would the copy and paste function come in handy? Well, it turns out that you can edit more than one layer at a
time and copy/paste features between layers. Why would we want to do this? Say we need to do some work on a
new layer but only need one or two lakes, not the 5,000 on our big_lakes layer. We can create a new layer and
use copy/paste to plop the needed lakes into it.
As an example, we will copy some lakes to a new layer:
1. Load the layer you want to copy from (source layer)
2. Load or create the layer you want to copy to (target layer)
3. Start editing for target layer
4. Make the source layer active by clicking on it in the legend
5. Use the Select Single Feature tool to select the feature(s) on the source layer
6. Click on the Copy Features tool
7. Make the destination layer active by clicking on it in the legend
8. Click on the Paste Features tool
9. Stop editing and save the changes
What happens if the source and target layers have different schemas (field names and types are not the same)?
QGIS populates what matches and ignores the rest. If you don’t care about the attributes being copied to the target
layer, it doesn’t matter how you design the fields and data types. If you want to make sure everything - the feature
and its attributes - gets copied, make sure the schemas match.
Truco: Congruency of Pasted Features
If your source and destination layers use the same projection, then the pasted features will have geometry identical
to the source layer. However, if the destination layer is a different projection, then QGIS cannot guarantee the ge-
ometry is identical. This is simply because there are small rounding-off errors involved when converting between
projections.
Truco: Copy string attribute into another
If you have created a new column in your attribute table with type ‘string’ and want to paste values from another
attribute column that has a greater length the length of the column size will be extended to the same amount. This
is because the GDAL Shapefile driver starting with GDAL/OGR 1.10 knows to auto-extend string and integer
fields to dynamically accomodate for the length of the data to be inserted.
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Deleting Selected Features
If we want to delete an entire polygon, we can do that by first selecting the polygon using the regular
Select Single Feature tool. You can select multiple features for deletion. Once you have the selection set, use the
Delete Selected tool to delete the features.
The Cut Features tool on the digitizing toolbar can also be used to delete features. This effectively deletes the
feature but also places it on a “spatial clipboard”. So, we cut the feature to delete. We could then use the
Paste Features tool to put it back, giving us a one-level undo capability. Cut, copy, and paste work on the currently
selected features, meaning we can operate on more than one at a time.
Saving Edited Layers
When a layer is in editing mode, any changes remain in the memory of QGIS. Therefore, they are not commit-
ted/saved immediately to the data source or disk. If you want to save edits to the current layer but want to continue
editing without leaving the editing mode, you can click the Save Layer Edits button. When you turn editing mode
off with Toggle editing (or quit QGIS for that matter), you are also asked if you want to save your changes or
discard them.
If the changes cannot be saved (e.g., disk full, or the attributes have values that are out of range), the QGIS
in-memory state is preserved. This allows you to adjust your edits and try again.
Truco: Data Integrity
It is always a good idea to back up your data source before you start editing. While the authors of QGIS have
made every effort to preserve the integrity of your data, we offer no warranty in this regard.
12.5.5 Advanced digitizing
Icon Purpose Icon Purpose
Undo Redo
Rotate Feature(s) Simplify Feature
Add Ring Add Part
Fill Ring Delete Ring
Delete Part Reshape Features
Offset Curve Split Features
Split Parts Merge Selected Features
Merge Attributes of Selected Features Rotate Point Symbols
Table Advanced Editing: Vector layer advanced editing toolbar
Undo and Redo
The Undo and Redo tools allows you to undo or redo vector editing operations. There is also a dockable
widget, which shows all operations in the undo/redo history (see Figure_edit_3). This widget is not displayed by
default; it can be displayed by right clicking on the toolbar and activating the Undo/Redo checkbox. Undo/Redo
is however active, even if the widget is not displayed.
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Figure 12.42: Redo and Undo digitizing steps
When Undo is hit, the state of all features and attributes are reverted to the state before the reverted operation
happened. Changes other than normal vector editing operations (for example, changes done by a plugin), may or
may not be reverted, depending on how the changes were performed.
To use the undo/redo history widget, simply click to select an operation in the history list. All features will be
reverted to the state they were in after the selected operation.
Rotate Feature(s)
Use Rotate Feature(s) to rotate one or multiple features in the map canvas. Press the Rotate Feature(s) icon and then
click on the feature to rotate. Either click on the map to place the rotated feature or enter an angle in the user input
widget. If you want to rotate several features, they shall be selected first.
If you enable the map tool with feature(s) selected, its (their) centroid appears and will be the rotation anchor
point. If you want to move the anchor point, hold the Ctrl button and click on the map to place it.
If you hold Shift before clicking on the map, the rotation will be done in 45 degree steps, which can be modified
afterwards in the user input widget.
Simplify Feature
The Simplify Feature tool allows you to reduce the number of vertices of a feature, as long as the geometry
doesn’t change. With the tool you can also simplify multi-part features. First, drag a rectangle over the feature.
The vertices will be highlighted in red while the color of the feature will change and a dialog where you can define
a tolerance in map units or pixels will appear. QGIS calculates the amount of vertices that can be deleted while
maintaining the geometry using the given tolerance. The higher the tolerance is the more vertices can be deleted.
After gaining the statistics about the simplification just klick the OK button. The tolerance you used will be saved
when leaving a project or when leaving an edit session. So you can go back to the same tolerance the next time
when simplifying a feature.
Add Ring
You can create ring polygons using the Add Ring icon in the toolbar. This means that inside an existing area, it
is possible to digitize further polygons that will occur as a ‘hole’, so only the area between the boundaries of the
outer and inner polygons remains as a ring polygon.
Add Part
You can add part polygons to a selected multipolygon. The new part polygon must be digitized outside the
selected multi-polygon.
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Fill Ring
You can use the Fill Ring function to add a ring to a polygon and add a new feature to the layer at the same time.
Thus you need not first use the Add Ring icon and then the Add feature function anymore.
Delete Ring
The Delete Ring tool allows you to delete ring polygons inside an existing area. This tool only works with
polygon layers. It doesn’t change anything when it is used on the outer ring of the polygon. This tool can be used
on polygon and multi-polygon features. Before you select the vertices of a ring, adjust the vertex edit tolerance.
Delete Part
The Delete Part tool allows you to delete parts from multifeatures (e.g., to delete polygons from a multi-polygon
feature). It won’t delete the last part of the feature; this last part will stay untouched. This tool works with all
multi-part geometries: point, line and polygon. Before you select the vertices of a part, adjust the vertex edit
tolerance.
Reshape Features
You can reshape line and polygon features using the Reshape Features icon on the toolbar. It replaces the line or
polygon part from the first to the last intersection with the original line. With polygons, this can sometimes lead
to unintended results. It is mainly useful to replace smaller parts of a polygon, not for major overhauls, and the
reshape line is not allowed to cross several polygon rings, as this would generate an invalid polygon.
For example, you can edit the boundary of a polygon with this tool. First, click in the inner area of the polygon
next to the point where you want to add a new vertex. Then, cross the boundary and add the vertices outside the
polygon. To finish, right-click in the inner area of the polygon. The tool will automatically add a node where the
new line crosses the border. It is also possible to remove part of the area from the polygon, starting the new line
outside the polygon, adding vertices inside, and ending the line outside the polygon with a right click.
Nota: The reshape tool may alter the starting position of a polygon ring or a closed line. So, the point that is
represented ‘twice’ will not be the same any more. This may not be a problem for most applications, but it is
something to consider.
Offset Curves
The Offset Curve tool creates parallel shifts of line layers. The tool can be applied to the edited layer (the
geometries are modified) or also to background layers (in which case it creates copies of the lines / rings and adds
them to the the edited layer). It is thus ideally suited for the creation of distance line layers. The displacement is
shown at the bottom left of the taskbar.
To create a shift of a line layer, you must first go into editing mode and activate the Offset Curve tool. Then click
on a feature to shift it. Move the mouse and click where wanted or enter the desired distance in the user input
widget. Your changes may then be saved with the|mActionSaveEdits|:sup:Save Layer Edits tool.
QGIS options dialog (Digitizing tab then Curve offset tools section) allows you to configure some parameters
like Join style,Quadrant segments,Miter limit.
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Split Features
You can split features using the Split Features icon on the toolbar. Just draw a line across the feature you want to
split.
Split parts
In QGIS 2.0 it is now possible to split the parts of a multi part feature so that the number of parts is increased. Just
draw a line across the part you want to split using the Split Parts icon.
Merge selected features
The Merge Selected Features tool allows you to merge features. A new dialog will allow you to choose which value
to choose between each selected features or select a function (Minimum, Maximum, Median, Sum, Skip Attribute)
to use for each column. If features don’t have a common boundaries, a multipolygon will be created.
Merge attributes of selected features
The Merge Attributes of Selected Features tool allows you to merge attributes of features with common boundaries
and attributes without merging their boundaries. First, select several features at once. Then press the
Merge Attributes of Selected Features button. Now QGIS asks you which attributes are to be applied to all selected objects.
As a result, all selected objects have the same attribute entries.
Rotate Point Symbols
Rotate Point Symbols allows you to change the rotation of point symbols in the map canvas. You must first define
a rotation column from the attribute table of the point layer in the Advanced menu of the Style menu of the Layer
Properties. Also, you will need to go into the ‘SVG marker’ and choose Data defined properties .... Activate
Angle and choose ‘rotation’ as field. Without these settings, the tool is inactive.
Figure 12.43: Rotate Point Symbols
To change the rotation, select a point feature in the map canvas and rotate it, holding the left mouse button pressed.
A red arrow with the rotation value will be visualized (see Figure_edit_4). When you release the left mouse button
again, the value will be updated in the attribute table.
Nota: If you hold the Ctrl key pressed, the rotation will be done in 15 degree steps.
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12.5.6 The Advanced Digitizing panel
When capturing new geometries or geometry parts you also have the possibility to use the Advanced Digitizing
panel. You can digitize lines exactly parallel or at a specific angle or lock lines to specific angles. Furthermore
you can enter coordinates directly so that you can make a precise definition for your new geomtry.
_figure_advanced_edit 1:
Figure 12.44: The Advanced Digitizing panel
The tools are not enabled if the map view is in geographic coordinates.
12.5.7 Creating new Vector layers
QGIS allows you to create new shapefile layers, new SpatiaLite layers, new GPX layers and New Temporary
Scratch Layers. Creation of a new GRASS layer is supported within the GRASS plugin. Please refer to section
Creating a new GRASS vector layer for more information on creating GRASS vector layers.
Creating a new Shapefile layer
To create a new shape layer for editing, choose New New Shapefile Layer... from the Layer menu. The
New Vector Layer dialog will be displayed as shown in Figure_edit_5. Choose the type of layer (point, line or
polygon) and the CRS (coordinate reference system).
Note that QGIS does not yet support creation of 2.5D features (i.e., features with X,Y,Z coordinates).
To complete the creation of the new shapefile layer, add the desired attributes by clicking on the [Add to attributes
list] button and specifying a name and type for the attribute. A first ‘id’ column is added as default but can be
removed, if not wanted. Only Type: real ,Type: integer ,Type: string and Type:date
attributes are supported. Additionally and according to the attribute type, you can also define the width and
precision of the new attribute column. Once you are happy with the attributes, click [OK] and provide a name
for the shapefile. QGIS will automatically add a .shp extension to the name you specify. Once the layer has
been created, it will be added to the map, and you can edit it in the same way as described in section Digitizing an
existing layer above.
Creating a new SpatiaLite layer
To create a new SpatiaLite layer for editing, choose New New SpatiaLite Layer... from the Layer menu.
The New SpatiaLite Layer dialog will be displayed as shown in Figure_edit_6.
The first step is to select an existing SpatiaLite database or to create a new SpatiaLite database. This can be done
with the browse button to the right of the database field. Then, add a name for the new layer, define the
layer type, and specify the coordinate reference system with [Specify CRS]. If desired, you can select Create
an autoincrementing primary key.
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Figure 12.45: Creating a new Shapefile layer Dialog
Figure 12.46: Creating a New SpatiaLite layer Dialog
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To define an attribute table for the new SpatiaLite layer, add the names of the attribute columns you want to create
with the corresponding column type, and click on the [Add to attribute list] button. Once you are happy with the
attributes, click [OK]. QGIS will automatically add the new layer to the legend, and you can edit it in the same
way as described in section Digitizing an existing layer above.
Further management of SpatiaLite layers can be done with the DB Manager. See Complemento administrador de
BBDD.
Creating a new GPX layer
To create a new GPX file, you need to load the GPS plugin first. Plugins Plugin Manager... opens the
Plugin Manager Dialog. Activate the GPS Tools checkbox.
When this plugin is loaded, choose New Create new GPX Layer... from the Layer menu. In the Save new
GPX file as dialog, you can choose where to save the new GPX layer.
Creating a new Temporary Scratch Layer
Empty, editable memory layers can be defined using Layer Create Layer New Temporary Scratch Layer.
Here you can even create Multipoint,Multiline and Multipolygon Layers beneath Point,Line and
Polygon Layers. Temporary Scratch Layers are not saved and will be discarded when QGIS is closed. See also
paste_into_layer .
12.5.8 Working with the Attribute Table
The attribute table displays features of a selected layer. Each row in the table represents one map feature, and
each column contains a particular piece of information about the feature. Features in the table can be searched,
selected, moved or even edited.
To open the attribute table for a vector layer, make the layer active by clicking on it in the map legend area. Then,
from the main Layer menu, choose Open Attribute Table. It is also possible to right click on the layer and
choose Open Attribute Table from the drop-down menu, and to click on the Open Attribute Table button
in the Attributes toolbar.
This will open a new window that displays the feature attributes for the layer (figure_attributes_1). The number
of features and the number of selected features are shown in the attribute table title.
Figure 12.47: Attribute Table for regions layer
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Selecting features in an attribute table
Each selected row in the attribute table displays the attributes of a selected feature in the layer. If the set of
features selected in the main window is changed, the selection is also updated in the attribute table. Likewise, if
the set of rows selected in the attribute table is changed, the set of features selected in the main window will be
updated.
Rows can be selected by clicking on the row number on the left side of the row. Multiple rows can be marked by
holding the Ctrl key. A continuous selection can be made by holding the Shift key and clicking on several
row headers on the left side of the rows. All rows between the current cursor position and the clicked row are
selected. Moving the cursor position in the attribute table, by clicking a cell in the table, does not change the row
selection. Changing the selection in the main canvas does not move the cursor position in the attribute table.
The table can be sorted by any column, by clicking on the column header. A small arrow indicates the sort order
(downward pointing means descending values from the top row down, upward pointing means ascending values
from the top row down).
For a simple search by attributes on only one column, choose the Column filter from the menu in the bottom
left corner. Select the field (column) on which the search should be performed from the drop-down menu, and hit
the [Apply] button. Then, only the matching features are shown in the attribute table.
To make a selection, you have to use the Select features using an Expression icon on top of the attribute table.
Select features using an Expression allows you to define a subset of a table using a Function List like in the Field Calculator
(see Field Calculator). The query result can then be saved as a new vector layer. For example, if you want to find
regions that are boroughs from regions.shp of the QGIS sample data, you have to open the Fields and Values
menu and choose the field that you want to query. Double-click the field ‘TYPE_2’ and also [Load all unique
values] . From the list, choose and double-click ‘Borough’. In the Expression field, the following query appears:
"TYPE_2" =’Borough’
Here you can also use the Function list Recent (Selection) to make a selection that you used before. The
expression builder remembers the last 20 used expressions.
The matching rows will be selected, and the total number of matching rows will appear in the title bar of the
attribute table, as well as in the status bar of the main window. For searches that display only selected features on
the map, use the Query Builder described in section Constructor de consultas.
To show selected records only, use Show Selected Features from the menu at the bottom left.
The field calculator bar allows you to make calculations on the selected rows only. For example, you can alter the
number of the ID field of the file:regions.shp with the expression
ID+5
as shown in figure_attributes_1 .
The other buttons at the top of the attribute table window provide the following functionality:
Toggle editing mode to edit single values and to enable functionalities described below (also with Ctrl+E)
Save Edits (also with Ctrl+S)
Unselect all (also with Ctrl+U)
Move selected to top (also with Ctrl+T)
Invert selection (also with Ctrl+R)
Copy selected rows to clipboard (also with Ctrl+C)
Zoom map to the selected rows (also with Ctrl+J)
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Pan map to the selected rows (also with Ctrl+P)
Delete selected features (also with Ctrl+D)
New Column for PostGIS layers and for OGR layers with GDAL version >= 1.6 (also with Ctrl+W)
Delete Column for PostGIS layers and for OGR layers with GDAL version >= 1.9 (also with Ctrl+L)
Open field calculator (also with Ctrl+I)
Below these buttons is the Field Calculator bar, which allows calculations to be quickly applied attributes visible
in the table. This bar uses the same expressions as the Field Calculator (see Field Calculator).
Truco: Skip WKT geometry
If you want to use attribute data in external programs (such as Excel), use the Copy selected rows to clipboard button.
You can copy the information without vector geometries if you deactivate Settings Options Data sources
menu Copy geometry in WKT representation from attribute table.
Save selected features as new layer
The selected features can be saved as any OGR-supported vector format and also transformed into another coordi-
nate reference system (CRS). Just open the right mouse menu of the layer and click on Save as to define the name
of the output file, its format and CRS (see section Leyenda del mapa). To save the selection ensure that the
Save only selected features is selected. It is also possible to specify OGR creation options within the dialog.
Paste into new layer
Features that are on the clipboard may be pasted into a new layer. To do this, first make a layer editable. Select
some features, copy them to the clipboard, and then paste them into a new layer using Edit Paste Features as
and choosing New vector layer or New memory layer.
This applies to features selected and copied within QGIS and also to features from another source defined using
well-known text (WKT).
Working with non spatial attribute tables
QGIS allows you also to load non-spatial tables. This currently includes tables supported by OGR and delimited
text, as well as the PostgreSQL, MSSQL and Oracle provider. The tables can be used for field lookups or just
generally browsed and edited using the table view. When you load the table, you will see it in the legend field. It
can be opened with the Open Attribute Table tool and is then editable like any other layer attribute table.
As an example, you can use columns of the non-spatial table to define attribute values, or a range of values that are
allowed, to be added to a specific vector layer during digitizing. Have a closer look at the edit widget in section
Fields Menu to find out more.
12.5.9 Creating one to many relations
Relations are a technique often used in databases. The concept is, that features (rows) of different layers (tables)
can belong to each other.
As an example you have a layer with all regions of alaska (polygon) which provides some attributes about its name
and region type and a unique id (which acts as primary key).
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Foreign keys
Then you get another point layer or table with information about airports that are located in the regions and you
also want to keep track of these. If you want to add them to the region layer, you need to create a one to many
relation using foreign keys, because there are several airports in most regions.
Figure 12.48: Alaska region with airports
In addition to the already existing attributes in the airports attribute table another field fk_region which acts as a
foreign key (if you have a database, you will probably want to define a constraint on it).
This field fk_region will always contain an id of a region. It can be seen like a pointer to the region it belongs
to. And you can design a custom edit form for the editing and QGIS takes care about the setup. It works with
different providers (so you can also use it with shape and csv files) and all you have to do is to tell QGIS the
relations between your tables.
Layers
QGIS makes no difference between a table and a vector layer. Basically, a vector layer is a table with a geometry.
So can add your table as a vector layer. To demostrate you can load the ‘region’ shapefile (with geometries) and
the ‘airport’ csv table (without geometries) and a foreign key (fk_region) to the layer region. This means, that
each airport belongs to exactly one region while each region can have any number of airports (a typical one to
many relation).
Definition (Relation Manager)
The first thing we are going to do is to let QGIS know about the relations between the layer. This is done in
Settings Project Properties. Open the Relations menu and click on Add.
name is going to be used as a title. It should be a human readable string, describing, what the relation is
used for. We will just call say “Airports” in this case.
referencing layer is the one with the foreign key field on it. In our case this is the airports layer
referencing field will say, which field points to the other layer so this is fk_region in this case
referenced layer is the one with the primary key, pointed to, so here it is the regions layer
referenced field is the primary key of the referenced layer so it is ID
id will be used for internal purposes and has to be unique. You may need it to build custom forms once this
is supported. If you leave it empty, one will be generated for you but you can assign one yourself to get one
that is easier to handle.
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Figure 12.49: Relation Manager
Forms
Now that QGIS knows about the relation, it will be used to improve the forms it generates. As we did not change
the default form method (autogenerated) it will just add a new widget in our form. So let’s select the layer region
in the legend and use the identify tool. Depending on your settings, the form might open directly or you will have
to choose to open it in the identification dialog under actions.
Figure 12.50: Identification dialog regions with relation to airports
As you can see, the airports assigned to this particular region are all shown in a table. And there are also some
buttons available. Let’s review them shortly
The button is for toggling the edit mode. Be aware that it toggles the edit mode of the airport layer,
although we are in the feature form of a feature from the region layer. But the table is representing features
of the airport layer.
The button will add a new feature to the airport layer. And it will assign the new airport to the current
region by default.
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The button will delete the selected airport permanently.
The symbol will open a new dialog where you can select any existing airport which will then be assigned
to the current region. This may be handy if you created the airport on the wrong region by accident.
The symbol will unlink the selected airport from the current region, leaving them unassigned (the
foreign key is set to NULL) effectively.
The two buttons to the right switch between table view and form view where the later let’s you view all the
airports in their respective form.
If you work on the airport table, a new widget type is available which lets you embed the feature form of the
referenced region on the feature form of the airports. It can be used when you open the layer properties of the
airports table, switch to the Fields menu and change the widget type of the foreign key field ‘fk_region’ to Relation
Reference.
If you look at the feature dialog now, you will see, that the form of the region is embedded inside the airports form
and will even have a combobox, which allows you to assign the current airport to another region.
Figure 12.51: Identification dialog airport with relation to regions
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12.6 Constructor de consultas
El Constructor de consultas permite definir un sub conjunto de una tabla utilizando SQL- como clausulas WHERE
y visualizar los resultados en la ventana principal. El resultado de la consulta se puede guardar como una nueva
capa vectorial.
12.6.1 Consulta
Abra el Constructor de consultas al abrir las Propiedades de la capa y vaya al menú General. Bajo Subconjunto
de datos espaciales, haga clic en el botón [Constructor de consultas] para abrir el Constructor de consultas.
Por ejemplo, si tiene una capa de regiones con un campo TYPE_2, podría seleccionar sólo regiones que
estén en municipio y la caja Expresión de filtrado específica por el proveedor del Constructor de consultas.
Figure_attributes_2 muestra un ejemplo de Constructor de consultas poblada con la capa regions.shp de los
datos de ejemplo de QGIS. Las secciones de campos, valores y operadores ayudan a construir el SQL- como
consulta.
La Lista de campos contiene todos las columnas de atributos de la tabla de atributos a ser buscados. Para agregar
una columna de atributos al campo de la clausula SQL WHERE, haga doble clic en el nombre de la lista de
campos. En general puede usar varios campos, valores y operadores para construir la consulta, o simplemente
puede escribirlo en la caja SQL.
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Figure 12.52: Constructor de consultas
La Lista de valores lista los valores de una tabla de atributos. Para listar todos los valores posibles de un atributo,
seleccione el atributo en la lista de campos y haga clic en el botón [Todos]. Para listar los primeros 25 valores
únicos de una columna de atributos, seleccione la columna de atributos en la lista de campos y haga clic en el
botón [Muestra]. Para añadir un valor al campo de la clausula WHERE de SQL, haga doble clic en el nombre en
la lista de valores.
La Sección de Operadores contiene todos los operadores utilizables. Para añadir un operador al campo de la
clausula WHERE, haga clic en el botón correspondiente. Los operadores relacionales ( =,>, ...), operador de
comparación de cadenas (COMO), y los operadores lógicos (Y,O, ...) están disponibles.
El botón [Probar] muestra un cuadro de mensaje con el numero de objetos espaciales que satisfacen la consulta
actual, que es útil en el proceso de construcción de consultas. El botón [Limpiar] limpia el texto en el campo de
texto de la clausula WHERE de SQL. El botón [Aceptar] cierra la ventana y selecciona los objetos espaciales que
satisfacen la consulta. El botón [Cancelar] cierra la ventana sin cambiar la selección actual.
QGIS trata los actos de subconjunto resultantes como si donde toda la capa. Por ejemplo, si aplica el filtro por
encima de ‘Borough’, no se puede desplegar, consultar, guardar o editar Ankorage, porque eso es un ‘Municipality’
y por lo tanto no forma parte del subconjunto.
La única excepción es que a menos que su capa sea parte de una base de datos, utilizar un subconjunto le impedirá
la edición de la capa.
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12.7 Field Calculator
The Field Calculator button in the attribute table allows you to perform calculations on the basis of existing
attribute values or defined functions, for instance, to calculate length or area of geometry features. The results can
be written to a new attribute field, a virtual field, or they can be used to update values in an existing field.
Truco: Virtual Fields
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Virtual fields are not permanent and are not saved.
To make a field virtual it must be done when the field is made.
The field calculator is now available on any layer that supports edit. When you click on the field calculator icon
the dialog opens (see figure_attributes_3). If the layer is not in edit mode, a warning is displayed and using the
field calculator will cause the layer to be put in edit mode before the calculation is made.
The quick field calculation bar on top of the attribute table is only visible if the layer is editable.
In quick field calculation bar, you first select the existing field name then open the expression dialog to create your
expression or write it directly in the field then click on Update All button.
12.7.1 Expression tab
In the field calculator dialog, you first must select whether you want to only update selected features, create a new
attribute field where the results of the calculation will be added or update an existing field.
Figure 12.53: Field Calculator
If you choose to add a new field, you need to enter a field name, a field type (integer, real or string), the total field
width, and the field precision (see figure_attributes_3). For example, if you choose a field width of 10 and a field
precision of 3, it means you have 6 digits before the dot, then the dot and another 3 digits for the precision.
A short example illustrates how field calculator works when using the Expression tab. We want to calculate the
length in km of the railroads layer from the QGIS sample dataset:
1. Load the shapefile railroads.shp in QGIS and press Open Attribute Table.
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2. Click on Toggle editing mode and open the Field Calculator dialog.
3. Select the Create a new field checkbox to save the calculations into a new field.
4. Add length as Output field name and real as Output field type, and define Output field width to be 10
and Precision, 3.
5. Now double click on function $length in the Geometry group to add it into the Field calculator expression
box.
6. Complete the expression by typing ‘’/ 1000” in the Field calculator expression box and click [Ok].
7. You can now find a new field length in the attribute table.
The available functions are listed in Expressions chapter.
12.7.2 Function Editor tab
With the Function Editor you are able to define your own Python custom functions in a comfortable way. The
function editor will create new Python files in qgis2pythonexpressions and will auto load all functions
defined when starting QGIS. Be aware that new functions are only saved in the expressions folder and not in
the project file. If you have a project that uses one of your custom functions you will need to also share the .py file
in the expressions folder.
Here’s a short example on how to create your own functions:
@qgsfunction(args="auto", group=’Custom’)
def myfunc(value1, value2 feature, parent):
pass
The short example creates a function ‘myfunc’ that will give you a function with two values. When using the
args=’auto’ function argument the number of function arguments required will be calculated by the number of
arguments the function has been defined with in Python (minus 2 - feature, and parent).
This function then can be used with the following expression:
myfunc(’test1’,’test2’)
Your function will be implemented in the ‘Custom’ Functions of the Expression tab after using the Run Script
button.
Further information about creating Python code can be found on http://www.qgis.org/html/en/docs/pyqgis_developer_cookbook/index.html.
The function editor is not only limited to working with the field calculator, it can be found whenever you work
with expressions. See also Expressions.
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CHAPTER 13
Trabajar con catos raster
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13.1 Working with Raster Data
This section describes how to visualize and set raster layer properties. QGIS uses the GDAL library to read and
write raster data formats, including ArcInfo Binary Grid, ArcInfo ASCII Grid, GeoTIFF, ERDAS IMAGINE, and
many more. GRASS raster support is supplied by a native QGIS data provider plugin. The raster data can also be
loaded in read mode from zip and gzip archives into QGIS.
As of the date of this document, more than 100 raster formats are supported by the GDAL library
(see GDAL-SOFTWARE-SUITE in Referencias bibliográficas y web). A complete list is available at
http://www.gdal.org/formats_list.html.
Nota: Not all of the listed formats may work in QGIS for various reasons. For example, some require external
commercial libraries, or the GDAL installation of your OS may not have been built to support the format you want
to use. Only those formats that have been well tested will appear in the list of file types when loading a raster into
QGIS. Other untested formats can be loaded by selecting the [GDAL] All files (*)filter.
Working with GRASS raster data is described in section GRASS GIS Integration.
13.1.1 What is raster data?
Raster data in GIS are matrices of discrete cells that represent features on, above or below the earth’s surface. Each
cell in the raster grid is the same size, and cells are usually rectangular (in QGIS they will always be rectangular).
Typical raster datasets include remote sensing data, such as aerial photography, or satellite imagery and modelled
data, such as an elevation matrix.
Unlike vector data, raster data typically do not have an associated database record for each cell. They are geocoded
by pixel resolution and the x/y coordinate of a corner pixel of the raster layer. This allows QGIS to position the
data correctly in the map canvas.
QGIS makes use of georeference information inside the raster layer (e.g., GeoTiff) or in an appropriate world file
to properly display the data.
13.1.2 Loading raster data in QGIS
Raster layers are loaded either by clicking on the Add Raster Layer icon or by selecting the Layer Add
Raster Layer menu option. More than one layer can be loaded at the same time by holding down the Ctrl or
Shift key and clicking on multiple items in the Open a GDAL Supported Raster Data Source dialog.
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Once a raster layer is loaded in the map legend, you can click on the layer name with the right mouse button to
select and activate layer-specific features or to open a dialog to set raster properties for the layer.
Right mouse button menu for raster layers
Zoom to Layer Extent
Zoom to Best Scale (100%)
Stretch Using Current Extend
Show in Overview
Remove
Duplicate
Set Layer CRS
Set Project CRS from Layer
Save as ...
Properties
Rename
Copy Style
Add New Group
Expand all
Collapse all
Update Drawing Order
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13.2 Raster Properties Dialog
To view and set the properties for a raster layer, double click on the layer name in the map legend, or right click on
the layer name and choose Properties from the context menu. This will open the Raster Layer Properties dialog
(see figure_raster_1).
There are several menus in the dialog:
General
Style
Transparency
Pyramids
Histogram
Metadata
13.