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Ebenen laden

Let’s open some layers with data. QGIS recognizes vector and raster layers. Additionally, custom layer types are available, but we are not going to discuss them here.

Vector Layers

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!"

The data source identifier is a string and it is specific to each vector data provider. Layer’s name is used in the layer list widget. It is important to check whether the layer has been loaded successfully. If it was not, an invalid layer instance is returned.

The quickest way to open and display a vector layer in QGIS is the addVectorLayer function of the QgisInterface:

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

This creates a new layer and adds it to the map layer registry (making it appear in the layer list) in one step. The function returns the layer instance or None if the layer couldn’t be loaded.

The following list shows how to access various data sources using vector data providers:

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

    • for shapefile:

      vlayer = QgsVectorLayer("/path/to/shapefile/file.shp", "layer_name_you_like", "ogr")

    • for dxf (note the internal options in data source uri):

      uri = "/path/to/dxffile/file.dxf|layername=entities|geometrytype=Point"
vlayer = QgsVectorLayer(uri, "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(False), "layer name you like", "postgres")

    Bemerkung

    The False argument passed to uri.uri(False) prevents the expansion of the authentication configuration parameters, if you are not using any authentication configuration this argument does not make any difference.

  • 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” for 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")

    Bemerkung

    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 — 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(uri, "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))

Bemerkung

You can change the data source of an existing layer by calling setDataSource() on a QgsVectorLayer instance, as in the following example:

# layer is a vector layer, uri is a QgsDataSourceURI instance
layer.setDataSource(uri.uri(), "layer name you like", "postgres")

Raster Layers

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!"

Similarly to vector layers, raster layers can be loaded using the addRasterLayer function of the QgisInterface:

iface.addRasterLayer("/path/to/raster/file.tif", "layer name you like")

This creates a new layer and adds it to the map layer registry (making it appear in the layer list) in one step.

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')

detailed URI settings can be found in provider documentation

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://irs.gis-lab.info/?layers=landsat&styles=&format=image/jpeg&crs=EPSG:4326'
rlayer = QgsRasterLayer(urlWithParams, 'some layer name', 'wms')
if not rlayer.isValid():
  print "Layer failed to load!"

Map Layer Registry

If you would like to use the opened layers for rendering, do not forget to add them to map layer registry. The map layer registry takes ownership of layers and they can be later accessed from any part of the application by their unique ID. When the layer is removed from map layer registry, it gets deleted, too.

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()