QGIS Vector Layer – Getting Projection from Vector Layer in QGIS

coordinate systempyqgisqgisvector-layer

I am trying to set the projection on a raster to match that of a vector point layer. Thus I need to find out what is the projection of a given layer, to use it in the GDAL.Dataset.SetProjection() so that I can create the GeoTIFF with the appropriate projection.

How do I do that in QGIS using Python?

Best Answer

Short answer

qgis.utils.iface.activeLayer().crs().authid()
# returns: PyQt4.QtCore.QString(u'EPSG:26913')

Explanation

qgis.utils.iface.activeLayer() returns a reference to the active QgsMapLayer.

QgsMapLayer.crs() returns the crs or QgsCoordinateReferenceSystem for the layer.

QgsCoordinateReferenceSystem.authid() returns the Authority identifier for the crs as a QString.

However, this is assuming there is an active layer, it is of a vector type, and it has a valid crs. You will want to test for validity of those items before committing to reprojecting a raster.

If you are reprojecting, using GDAL.Dataset.SetProjection() will not suffice, since it will only assign a projection and not reproject (warp) the raster to the same as your vector layer.

Related Solutions

PyQGIS Vector Layer – Creating New Empty Vector Layer with PyQGIS

This does not work any longer in qgis3. See a slightly modified answer below.

Have a look at the Memory provider as described in PyQGIS Cookbook.

Memory provider is intended to be used mainly by plugin or 3rd party app developers. It does not store data on disk, allowing developers to use it as a fast backend for some temporary layers.

# To avoid 'QVariant' is not defined error
from PyQt4.QtCore import *
# create layer
vl = QgsVectorLayer("Point", "temporary_points", "memory")
pr = vl.dataProvider()
# Enter editing mode
vl.startEditing()
# add fields
pr.addAttributes( [ QgsField("name", QVariant.String),
               QgsField("age",  QVariant.Int),
               QgsField("size", QVariant.Double) ] )
# add a feature
fet = QgsFeature()
fet.setGeometry( QgsGeometry.fromPoint(QgsPoint(10,10)) )
fet.setAttributeMap( { 0 : QVariant("Johny"),
                  1 : QVariant(20),
                  2 : QVariant(0.3) } )
pr.addFeatures( [ fet ] )
# Commit changes
vl.commitChanges()

[GIS] Getting dataProvider from vector layer outside QGIS

You haven't setup your QGIS Python environment correctly.

At the start of your script you need to put these four lines in order to tell the QGIS libs where to look for providers.

qgishome = 'C:\OSGeo4W\apps\qgis-dev\'
app = QgsApplication([], True)
QgsApplication.setPrefixPath(qgishome, True)
QgsApplication.initQgis()

when you need to exit the QGIS session you should use:

QgsApplication.exitQgis()

I have a growing simple set of examples at https://github.com/NathanW2/pyqgis-playground which can help to understand the basics of getting something setup. See the canvas example

Best Answer

Related Solutions

PyQGIS Vector Layer – Creating New Empty Vector Layer with PyQGIS

[GIS] Getting dataProvider from vector layer outside QGIS

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