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Cargar capas

Vamos a abrir algunas capas con datos. QGIS reconoce capas vectoriales y ráster. Además, están disponibles tipos de capas personalizadas, pero no se va a discutir de ellas aquí.

Capas Vectoriales

To load a vector layer, specify layer’s data source identifier, name for the layer and provider’s name:

layer = QgsVectorLayer(data_source, layer_name, provider_name)
if not layer.isValid():
  print "Layer failed to load!"

El identificador de la fuente de datos es una cadena y se especifica a cada proveedor de datos vectoriales. El nombre de la capa se utiliza en el widget de la lista de capa. Es importante validar si la capa se ha cargado satisfactoriamente. Si no fue así, se devuelve una instancia de capa no válida.

La manera más rápida para abrir y desplegar una capa vectorial en QGIS es la función addVectorLayer del QgisInterface:

layer = iface.addVectorLayer("/path/to/shapefile/file.shp", "layer_name_you_like", "ogr")
if not layer:
  print "Layer failed to load!"

Esto crea una nueva capa y lo añade al registro de capa de mapa (haciendolo aparecer en la lista de capas) en un paso. La función regresa la instancia de la capa o Nada si la capa no puede cargarse.

La siguiente lista muestra cómo acceder a varias fuentes de datos utilizando los proveedores de datos vectoriales:

  • OGR library (shapefiles and many other file formats) — data source is the path to the file

    vlayer = QgsVectorLayer("/path/to/shapefile/file.shp", "layer_name_you_like", "ogr")
    
  • PostGIS database — data source is a string with all information needed to create a connection to PostgreSQL database. QgsDataSourceURI class can generate this string for you. Note that QGIS has to be compiled with Postgres support, otherwise this provider isn’t available.

    uri = QgsDataSourceURI()
    # set host name, port, database name, username and password
    uri.setConnection("localhost", "5432", "dbname", "johny", "xxx")
    # set database schema, table name, geometry column and optionally
    # subset (WHERE clause)
    uri.setDataSource("public", "roads", "the_geom", "cityid = 2643")
    
    vlayer = QgsVectorLayer(uri.uri(), "layer_name_you_like", "postgres")
    
  • CSV or other delimited text files — to open a file with a semicolon as a delimiter, with field “x” for x-coordinate and field “y” with y-coordinate you would use something like this

    uri = "/some/path/file.csv?delimiter=%s&xField=%s&yField=%s" % (";", "x", "y")
    vlayer = QgsVectorLayer(uri, "layer_name_you_like", "delimitedtext")
    

    Note: from QGIS version 1.7 the provider string is structured as a URL, so the path must be prefixed with file://. Also it allows WKT (well known text) formatted geometries as an alternative to “x” and “y” fields, and allows the coordinate reference system to be specified. For example

    uri = "file:///some/path/file.csv?delimiter=%s&crs=epsg:4723&wktField=%s" % (";", "shape")
    
  • GPX files — the “gpx” data provider reads tracks, routes and waypoints from gpx files. To open a file, the type (track/route/waypoint) needs to be specified as part of the url

    uri = "path/to/gpx/file.gpx?type=track"
    vlayer = QgsVectorLayer(uri, "layer_name_you_like", "gpx")
    
  • SpatiaLite database — supported from QGIS v1.1. Similarly to PostGIS databases, QgsDataSourceURI can be used for generation of data source identifier

    uri = QgsDataSourceURI()
    uri.setDatabase('/home/martin/test-2.3.sqlite')
    schema = ''
    table = 'Towns'
    geom_column = 'Geometry'
    uri.setDataSource(schema, table, geom_column)
    
    display_name = 'Towns'
    vlayer = QgsVectorLayer(uri.uri(), display_name, 'spatialite')
    
  • MySQL WKB-based geometries, through OGR — data source is the connection string to the table

    uri = "MySQL:dbname,host=localhost,port=3306,user=root,password=xxx|layername=my_table"
    vlayer = QgsVectorLayer( uri, "my_table", "ogr" )
    
  • WFS connection:. the connection is defined with a URI and using the WFS provider

    uri = "http://localhost:8080/geoserver/wfs?srsname=EPSG:23030&typename=union&version=1.0.0&request=GetFeature&service=WFS",
    vlayer = QgsVectorLayer("my_wfs_layer", "WFS")
    

    The uri can be created using the standard urllib library.

    params = {
        'service': 'WFS',
        'version': '1.0.0',
        'request': 'GetFeature',
        'typename': 'union',
        'srsname': "EPSG:23030"
    }
    uri = 'http://localhost:8080/geoserver/wfs?' + urllib.unquote(urllib.urlencode(params))
    

Capas ráster

For accessing raster files, GDAL library is used. It supports a wide range of file formats. In case you have troubles with opening some files, check whether your GDAL has support for the particular format (not all formats are available by default). To load a raster from a file, specify its file name and base name

fileName = "/path/to/raster/file.tif"
fileInfo = QFileInfo(fileName)
baseName = fileInfo.baseName()
rlayer = QgsRasterLayer(fileName, baseName)
if not rlayer.isValid():
  print "Layer failed to load!"

De manera similar que las capas vectoriales, ráster se pueden cargar utilizando la función addRasterLayer de la QgisInterface:

iface.addRasterLayer("/path/to/raster/file.tif", "layer_name_you_like")

Esto crea una capa y lo agrega al registro de capa de mapa (haciendolo aparecer en la lista de capas) en un paso.

Raster layers can also be created from a WCS service.

layer_name = 'modis'
uri = QgsDataSourceURI()
uri.setParam('url', 'http://demo.mapserver.org/cgi-bin/wcs')
uri.setParam("identifier", layer_name)
rlayer = QgsRasterLayer(str(uri.encodedUri()), 'my_wcs_layer', 'wcs')

Ajustes del URI detallado se pueden encontrar en documentación de proveedor

Alternatively you can load a raster layer from WMS server. However currently it’s not possible to access GetCapabilities response from API — you have to know what layers you want

urlWithParams = 'url=http://wms.jpl.nasa.gov/wms.cgi&layers=global_mosaic&styles=pseudo&format=image/jpeg&crs=EPSG:4326'
rlayer = QgsRasterLayer(urlWithParams, 'some layer name', 'wms')
if not rlayer.isValid():
  print "Layer failed to load!"

Registro de capa de mapa

Si desea utilizar las capas abiertas para la representación, no olvide de añadirlos al registro de capa de mapa. El registro capa de mapa asume la propiedad de las capas y se puede acceder más tarde desde cualquier parte de la aplicación por su ID único. Cuando se retira la capa de registro capa de mapa, que se elimina, también.

Adding a layer to the registry

QgsMapLayerRegistry.instance().addMapLayer(layer)

Layers are destroyed automatically on exit, however if you want to delete the layer explicitly, use

QgsMapLayerRegistry.instance().removeMapLayer(layer_id)

For a list of loaded layers and layer ids, use

QgsMapLayerRegistry.instance().mapLayers()
TODO:
More about map layer registry?