Chargement de couches¶

Couches vectorielles¶

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!"

L’identifiant de source de données est une chaîne de texte, spécifique à chaque type de fournisseur de données vectorielles. Le nom de la couche est utilisée dans le widget liste de couches. Il est important de vérifier si la couche a été chargée ou pas. Si ce n’était pas le cas, une instance de couche non valide est retournée.

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.

La liste suivante montre comment accéder à différentes sources de données provenant de différents fournisseurs de données vectorielles:

• 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")
  

  Note

  L’argument False passé à uri.uri(False) empêche l’expansion des paramètres du système d’authentification. si vous n’avez pas configuré d’authentification, cet argument n’a aucun effet.

• 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")
  

  Note

  Le fournisseur de chaîne est structuré comme une URL, donc le chemin doit être préfixé avec file://. Il permet aussi d’utiliser les géométries formatées en WKT (well-known text) à la place des champs x et y, et permet de spécifier le système de référence géographique. Par exemple :

  uri = "file:///some/path/file.csv?delimiter=%s&crs=epsg:4723&wktField=%s" % (";", "shape")
  

• Fichiers GPS — le fournisseur de données “gpx” lit les trajets, routes et points de passage d’un fichier gpx. Pour ouvrir un fichier, le type (trajet/route/point) doit être fourni dans l’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')
  

• Géométries MySQL basées sur WKB, avec OGR — la source des données est la chaîne de connexion à la table :

  uri = "MySQL:dbname,host=localhost,port=3306,user=root,password=xxx|layername=my_table"
  vlayer = QgsVectorLayer( uri, "my table", "ogr" )
  

• Connexion WFS : la connexion est définie par une URL et utilise le fournisseur de données WFS

  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")
  

  L’url peut être crée en utilisant la bibliothèque standard : “urllib”

  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))
  

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

Couches raster¶

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.

Les couches raster peuvent également être créées à partir d’un service WCS.

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

Vous pouvez aussi charger une couche raster à partir d’un serveur WMS. Il n’est cependant pas encore possible d’avoir accès à la réponse de GetCapabilities à partir de l’API — vous devez connaître les couches que vous voulez :

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()