Setting the project CRS to existing User Defined Coordinate System with PyQGIS

Question

I want to set the project CRS with Python in QGIS. If I set it to an EPSG code, it works as expected:

canvas = iface.mapCanvas()
selectedcrs = "EPSG:4326"
target_crs = QgsCoordinateReferenceSystem()
target_crs.createFromUserInput(selectedcrs)
canvas.setDestinationCrs(target_crs)

But when I want to set it to a User Defined Coordinate System, the project CRS is made empty, because its code is not in the predefined CRS list in QGIS, but it's in the User Defined Coordinate Systems list in QGIS.

canvas = iface.mapCanvas()
selectedcrs = "USER:100001"
target_crs = QgsCoordinateReferenceSystem()
target_crs.createFromUserInput(selectedcrs)
canvas.setDestinationCrs(target_crs)

Is there a way to set the project CRS to an existing User Defined Coordinate System? Or is there a way to get the definition of that existing User Defined Coordinate System with Python?

To be clear: I don't want to change the CRS of a layer. I want to change the CRS of the project.

I can select "USER:100001" and set it from the Project Properties, but I want to do that with Python.

In my full script the "USER:100001" comes from the Coordinate Reference System Selector (which also lists the User Defined Coordinate Systems) and use it with this code:

projSelector = QgsGenericProjectionSelector()
projSelector.exec_()
projSelector.selectedCrsId()
selectedcrs = projSelector.selectedAuthId()

The selectedcrs variable is stored as a setting and later used by the script I originally posted above.

Answer 1

In QGIS 3.0 it is very easy. For instance our EPSG is 2320:

QgsProject.instance().setCrs(QgsCoordinateReferenceSystem(2320))

Answer 2

Instead of AuthId, you could use the CrsId in this way:

from qgis.gui import QgsGenericProjectionSelector

projSelector = QgsGenericProjectionSelector()
projSelector.exec_()
crsId = projSelector.selectedCrsId()

target_crs = QgsCoordinateReferenceSystem()
target_crs.createFromId( crsId, QgsCoordinateReferenceSystem.InternalCrsId )
iface.mapCanvas().setDestinationCrs( target_crs )

Answer 3

This is more of a workaround but if you know what the well-known text is for your custom CRS, you could use something like the following in the Python console:

crs_wkt = 'PROJCS["OSGB_1936_British_National_Grid",GEOGCS["GCS_OSGB 1936",DATUM["OSGB_1936",SPHEROID["Airy_1830",6377563.396,299.3249646]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",49],PARAMETER["central_meridian",-2],PARAMETER["scale_factor",0.9996012717],PARAMETER["false_easting",400000],PARAMETER["false_northing",-100000],UNIT["Meter",1]]'
iface.mapCanvas().mapRenderer().setDestinationCrs(QgsCoordinateReferenceSystem(crs_wkt))

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To find out what your CRS is in WKT, assign a layer with your custom CRS then run the following in the Python console:

qgis.utils.iface.activeLayer().crs().toWkt()

Or run the following tool:

Processing Toolbox > GDAL/OGR > [GDAL] > Miscellaneous > Information

It will show you some information including the Layer SRS WKT.

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A one-liner method is to assign a layer with your custom CRS and use:

iface.mapCanvas().mapRenderer().setDestinationCrs(QgsCoordinateReferenceSystem(qgis.utils.iface.activeLayer().crs().toWkt()))