这一节中所有的代码，如果不在QGIS命令行中执行，都需要导入如下包库

from qgis.core import (
    QgsCoordinateReferenceSystem,
    QgsCoordinateTransform,
    QgsProject,
    QgsPointXY,
)

8. Projections Support

8.1. Coordinate reference systems

Coordinate reference systems (CRS) are encapsulated by the QgsCoordinateReferenceSystem class. Instances of this class can be created in several different ways:

specify CRS by its ID
```
# EPSG 4326 is allocated for WGS84
crs = QgsCoordinateReferenceSystem("EPSG:4326")
print(crs.isValid())
```
```
True
```
QGIS supports different CRS identifiers with the following formats:
- EPSG:<code> --- ID assigned by the EPSG organization - handled with createFromOgcWms()
- POSTGIS:<srid>--- ID used in PostGIS databases - handled with createFromSrid()
- INTERNAL:<srsid> --- ID used in the internal QGIS database - handled with createFromSrsId()
- PROJ:<proj> - handled with createFromProj()
- WKT:<wkt> - handled with createFromWkt()

If no prefix is specified, WKT definition is assumed.

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)
print(crs.isValid())

True

create an invalid CRS and then use one of the create* functions to initialize it. In the following example we use a Proj string to initialize the projection.
```
crs = QgsCoordinateReferenceSystem()
crs.createFromProj("+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs")
print(crs.isValid())
```
```
True
```

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 the QGIS Python console or developing a plugin you do not care: everything is already set up for you.

Accessing spatial reference system information:

crs = QgsCoordinateReferenceSystem("EPSG:4326")

print("QGIS CRS ID:", crs.srsid())
print("PostGIS SRID:", crs.postgisSrid())
print("Description:", crs.description())
print("Projection Acronym:", crs.projectionAcronym())
print("Ellipsoid Acronym:", crs.ellipsoidAcronym())
print("Proj String:", crs.toProj())
# check whether it's geographic or projected coordinate system
print("Is geographic:", crs.isGeographic())
# check type of map units in this CRS (values defined in QGis::units enum)
print("Map units:", crs.mapUnits())

Output:

QGIS CRS ID: 3452
PostGIS SRID: 4326
Description: WGS 84
Projection Acronym: longlat
Ellipsoid Acronym: EPSG:7030
Proj String: +proj=longlat +datum=WGS84 +no_defs
Is geographic: True
Map units: 6

8.2. CRS Transformation

You can do transformation between different spatial reference systems by using the QgsCoordinateTransform class. The easiest way to use it is to create a source and destination CRS and construct a QgsCoordinateTransform instance with them and the current project. 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("EPSG:4326")    # WGS 84
crsDest = QgsCoordinateReferenceSystem("EPSG:32633")  # WGS 84 / UTM zone 33N
transformContext = QgsProject.instance().transformContext()
xform = QgsCoordinateTransform(crsSrc, crsDest, transformContext)

# forward transformation: src -> dest
pt1 = xform.transform(QgsPointXY(18,5))
print("Transformed point:", pt1)

# inverse transformation: dest -> src
pt2 = xform.transform(pt1, QgsCoordinateTransform.ReverseTransform)
print("Transformed back:", pt2)

Output:

Transformed point: <QgsPointXY: POINT(832713.79873844375833869 553423.98688333143945783)>
Transformed back: <QgsPointXY: POINT(18 5)>