Coordinate reference systems (CRS) are encapsulated by QgsCoordinateReferenceSystem class. Instances of this class can be created by several different ways:
specificeren van CRS met zijn ID
# PostGIS SRID 4326 is allocated for WGS84
crs = QgsCoordinateReferenceSystem(4326, QgsCoordinateReferenceSystem.PostgisCrsId)
QGIS gebruikt drie verschillende ID’s voor elk referentiesysteem:
PostgisCrsId — code gebruikt in PostGIS databases.
InternalCrsId — interne QGIS code.
EpsgCrsId — code in de EPSG notatie
Indien niet anders gespecificeerd in tweede parameter, wordt standaard PostGIS SRID gebruikt.
specificeren van CRS door zijn 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 look up 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
print "QGIS CRS ID:", crs.srsid()
print "PostGIS SRID:", crs.srid()
print "EPSG ID:", crs.epsg()
print "Description:", crs.description()
print "Projection Acronym:", crs.projectionAcronym()
print "Ellipsoid Acronym:", crs.ellipsoidAcronym()
print "Proj4 String:", crs.toProj4()
# check whether it's geographic or projected coordinate system
print "Is geographic:", crs.geographicFlag()
# check type of map units in this CRS (values defined in QGis::units enum)
print "Map units:", crs.mapUnits()
You can do transformation between different spatial reference systems by using QgsCoordinateTransform class. The easiest way to use it is to create source and destination CRS and construct QgsCoordinateTransform instance with them. Then just repeatedly call transform() function to do the transformation. By default it does forward transformation, but it is capable to do also inverse transformation
crsSrc = QgsCoordinateReferenceSystem(4326) # WGS 84
crsDest = QgsCoordinateReferenceSystem(32633) # WGS 84 / UTM zone 33N
xform = QgsCoordinateTransform(crsSrc, crsDest)
# forward transformation: src -> dest
pt1 = xform.transform(QgsPoint(18,5))
print "Transformed point:", pt1
# inverse transformation: dest -> src
pt2 = xform.transform(pt1, QgsCoordinateTransform.ReverseTransform)
print "Transformed back:", pt2