PyQGIS Documentation - Linfiniti Geo Blog

6.1 Coordinate reference systems
CHAPTER
SIX
PROJECTIONS SUPPORT
Coordinate reference systems (CRS) are encapsulated by QgsCoordinateReferenceSystem class. Instances
of this class can be created by several different ways:
• specify CRS by its ID:
# PostGIS SRID 4326 is allocated for WGS84
crs = QgsCoordinateReferenceSystem(4326, QgsCoordinateReferenceSystem.PostgisCrsId)
QGIS uses three different IDs for every reference system:
– PostgisCrsId - IDs used within PostGIS databases.
– InternalCrsId - IDs internally used in QGIS database.
– EpsgCrsId - IDs assigned by the EPSG organization
If not specified otherwise in second parameter, PostGIS SRID is used by default.
• specify CRS by its well-known text (WKT):
wkt = ’GEOGCS["WGS84", DATUM["WGS84", SPHEROID["WGS84", 6378137.0, 298.257223563]],\
PRIMEM["Greenwich", 0.0], UNIT["degree",0.017453292519943295],\
AXIS["Longitude",EAST], AXIS["Latitude",NORTH]]’
crs = QgsCoordinateReferenceSystem(wkt)
• create invalid CRS and then use one of the create*() functions to initialize it. In following example we use
Proj4 string to initialize the projection:
crs = QgsCoordinateReferenceSystem()
crs.createFromProj4("+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs")
It’s wise to check whether creation (i.e. lookup in the database) of the CRS has been successful: isValid() must
return True.
Note that for initialization of spatial reference systems QGIS needs to lookup appropriate values in its internal
database srs.db. Thus in case you create an independent application you need to set paths correctly with
QgsApplication.setPrefixPath() otherwise it will fail to find the database. If you are running the commands
from QGIS python console or developing a plugin you do not care: everything is already set up for you.
Accessing spatial reference system information:
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