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Lagen laden

Laten we enkele lagen met gegevens openen. QGIS herkent vector- en rasterlagen. Aanvullend zijn anagepaste typen lagen beschikbaar, maar die zullen we hier niet bespreken.

Vectorlagen

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!"

De identificatie van de gegevensbron van de laag is een string en is specifiek voor elke vector gegevensprovider. De naam van de laag wordt gebruikt in de widget Lagenlijst. Het is belangrijk om te controleren of de laag met succes is geladen. Als dat niet zo was wordt een ongeldige instance van de laag teruggegeven.

De snelste manier om een vectorlaag te openen en weer te geven in QGIS is de functie addVectorLayer van QgisInterface:

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

Dit maakt een nieuwe laag en voegt die in één stap toe aan het register van de kaartlagen (waardoor het verschijnt in de lagenlijst). De functie geeft de instance van de laag terug of None als de laag niet kon worden geladen.

De volgende lijst geeft weer hoe toegang wordt verkregen tot verscheidene gegevensbronnen met behulp van vector gegevensproviders:

  • 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))
    

Rasterlagen

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!"

Soortgelijk aan vectorlagen kunnen rasterlagen worden geladen met behulp van de functie addRasterLayer van QgisInterface:

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

Dit maakt een nieuwe laag en voegt die in één stap toe aan het register van de kaartlagen (waardoor het verschijnt in de lagenlijst).

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')

gedetailleerde instellingen voor de URI kunnen worden gevonden 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://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!"

Register van kaartlagen

Als u de geopende lagen wilt gebruiken voor renderen, vergeet dan niet om ze toe te voegen aan het register van kaartlagen. Het register van kaartlagen wordt eigenaar van de lagen en er kan later toegang toe worden verkregen vanuit elk deel van de toepassing door middel van hun unieke ID. Als de laag wordt verwijderd uit het register van de kaartlagen, wordt hij ook verwijderd.

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?