Caricamento del vettore¶

The code snippets on this page needs the following imports:

import os # This is is needed in the pyqgis console also
from qgis.core import (
    QgsVectorLayer
)

Vettori
Raster
QgsProject instance

Apri alcuni layer con dati. QGIS riconosce vettori e raster. inoltre sono disponibili tipi di layer personalizzati ma non li discuteremo qui.

Vettori ¶

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

# get the path to the shapefile e.g. /home/project/data/ports.shp
path_to_ports_layer = os.path.join(QgsProject.instance().homePath(), "data", "ports", "ports.shp")

# The format is:
# vlayer = QgsVectorLayer(data_source, layer_name, provider_name)

vlayer = QgsVectorLayer(path_to_ports_layer, "Ports layer", "ogr")
if not vlayer.isValid():
    print("Layer failed to load!")

390/5000 L’identificativo dell’origine dati è una stringa ed è specifico per ciascun fornitore di dati vettoriali. Il nome del livello viene utilizzato nell’oggetto elenco livelli. È importante verificare se il layer è stato caricato correttamente. In caso contrario, viene restituita un’istanza di livello non valida.

For a geopackage vector layer:

# get the path to a geopackage  e.g. /home/project/data/data.gpkg
path_to_gpkg = os.path.join(QgsProject.instance().homePath(), "data", "data.gpkg")
# append the layername part
gpkg_places_layer = path_to_gpkg + "|layername=places"
# e.g. gpkg_places_layer = "/home/project/data/data.gpkg|layername=places"
vlayer = QgsVectorLayer(gpkg_places_layer, "Places layer", "ogr")
if not vlayer.isValid():
    print("Layer failed to load!")

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

vlayer = iface.addVectorLayer(path_to_ports_layer, "Ports layer", "ogr")
if not vlayer:
  print("Layer failed to load!")

This creates a new layer and adds it to the current QGIS project (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.

Il seguente elenco mostra come accedere a varie fonti di dati usando i fornitori di dati vettoriali:

OGR library (Shapefile 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")
```
Nota

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={}&xField={}&yField={}".format(";", "x", "y")
vlayer = QgsVectorLayer(uri, "layer name you like", "delimitedtext")
```
Nota

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={}&crs=epsg:4723&wktField={}".format(";", "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))

Nota

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

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

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 filename and display name:

# get the path to a tif file  e.g. /home/project/data/srtm.tif
path_to_tif = os.path.join(QgsProject.instance().homePath(), "data", "srtm.tif")
rlayer = QgsRasterLayer(path_to_tif, "SRTM layer name")
if not rlayer.isValid():
    print("Layer failed to load!")

To load a raster from a geopackage:

# get the path to a geopackage  e.g. /home/project/data/data.gpkg
path_to_gpkg = os.path.join(QgsProject.instance().homePath(), "data", "data.gpkg")
# gpkg_raster_layer = "GPKG:/home/project/data/data.gpkg:srtm"
gpkg_raster_layer = "GPKG:" + path_to_gpkg + ":srtm"

rlayer = QgsRasterLayer(gpkg_raster_layer, "layer name you like", "gdal")

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 object:

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

This creates a new layer and adds it to the current project (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')

Here is a description of the parameters that the WCS URI can contain:

WCS URI is composed of key=value pairs separated by &. It is the same format like query string in URL, encoded the same way. QgsDataSourceUri should be used to construct the URI to ensure that special characters are encoded properly.

url (required) : WCS Server URL. Do not use VERSION in URL, because each version of WCS is using different parameter name for GetCapabilities version, see param version.
identifier (required) : Coverage name
time (optional) : time position or time period (beginPosition/endPosition[/timeResolution])
format (optional) : Supported format name. Default is the first supported format with tif in name or the first supported format.
crs (optional) : CRS in form AUTHORITY:ID, e.g. EPSG:4326. Default is EPSG:4326 if supported or the first supported CRS.
username (optional) : Username for basic authentication.
password (optional) : Password for basic authentication.
IgnoreGetMapUrl (optional, hack) : If specified (set to 1), ignore GetCoverage URL advertised by GetCapabilities. May be necessary if a server is not configured properly.
InvertAxisOrientation (optional, hack) : If specified (set to 1), switch axis in GetCoverage request. May be necessary for geographic CRS if a server is using wrong axis order.
IgnoreAxisOrientation (optional, hack) : If specified (set to 1), do not invert axis orientation according to WCS standard for geographic CRS.
cache (optional) : cache load control, as described in QNetworkRequest::CacheLoadControl, but request is resend as PreferCache if failed with AlwaysCache. Allowed values: AlwaysCache, PreferCache, PreferNetwork, AlwaysNetwork. Default is AlwaysCache.

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!")

QgsProject instance ¶

If you would like to use the opened layers for rendering, do not forget to add them to the QgsProject instance. The QgsProject instance 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 the project, it gets deleted, too. Layers can be removed by the user in the QGIS interface, or via Python using the removeMapLayer() method.

Adding a layer to the current project is done using the addMapLayer() method:

QgsProject.instance().addMapLayer(rlayer)

To add a layer at an absolute position:

# first add the layer without showing it
QgsProject.instance().addMapLayer(rlayer, False)
# obtain the layer tree of the top-level group in the project
layerTree = iface.layerTreeCanvasBridge().rootGroup()
# the position is a number starting from 0, with -1 an alias for the end
layerTree.insertChildNode(-1, QgsLayerTreeLayer(rlayer))

If you want to delete the layer use the removeMapLayer() method:

# QgsProject.instance().removeMapLayer(layer_id)
QgsProject.instance().removeMapLayer(rlayer.id())

In the above code, the layer id is passed (you can get it calling the id() method of the layer), but you can also pass the layer object itself.

For a list of loaded layers and layer ids, use the mapLayers() method:

QgsProject.instance().mapLayers()

Caricamento del vettore¶

Vettori¶

Raster¶

QgsProject instance¶

Vettori ¶

Raster ¶

QgsProject instance ¶