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QgsCoordinateReferenceSystem (QgsCRS)

The QgsCoordinateReferenceSystem (often abbreviated as QgsCRS) class in PyQGIS is used to represent and manipulate Coordinate Reference Systems (CRS). A CRS defines how the two-dimensional, projected map in your GIS relates to real places on the earth.

🔑 Key Responsibilities

  • Define and store coordinate reference system information.
  • Support for both EPSG codes and PROJ strings.
  • Conversion between different CRS definitions.
  • Check validity of CRS.
  • Provide information about units, ellipsoid, and projections.

📌 Creating a CRS

From EPSG Code

crs = QgsCoordinateReferenceSystem("EPSG:4326")
print(crs.isValid())  # True if valid

From PROJ String

crs = QgsCoordinateReferenceSystem.fromProj("+proj=longlat +datum=WGS84 +no_defs")

From WKT (Well-Known Text)

wkt = 'GEOGCS["WGS 84",DATUM["WGS_1984",...]]'
crs = QgsCoordinateReferenceSystem()
crs.createFromWkt(wkt)

📖 Useful Methods

Method Description
isValid() Checks if the CRS is valid
authid() Returns the authority ID (e.g., "EPSG:4326")
description() Returns a human-readable description
mapUnits() Returns units of measurement (e.g., degrees, meters)
toWkt() Converts CRS to WKT format
toProj() Returns PROJ string
ellipsoidAcronym() Returns ellipsoid name/acronym

⚙️ Example Usage

# Load CRS by EPSG code
crs = QgsCoordinateReferenceSystem("EPSG:32638")

if crs.isValid():
    print("CRS is valid")
    print("Auth ID:", crs.authid())
    print("Description:", crs.description())
    print("Units:", crs.mapUnits())

Output: 

CRS is valid
Auth ID: EPSG:32638
Description: WGS 84 / UTM zone 38N
Units: Meters

🔄 Reprojecting Layers to Another CRS

from qgis.core import QgsProject, QgsVectorLayer, QgsCoordinateTransform, QgsCoordinateReferenceSystem

# Load a vector layer
layer = QgsVectorLayer("path/to/shapefile.shp", "My Layer", "ogr")

# Define source and destination CRS
src_crs = layer.crs()
dest_crs = QgsCoordinateReferenceSystem("EPSG:4326")

# Setup transformation
transform = QgsCoordinateTransform(src_crs, dest_crs, QgsProject.instance())

# Transform a point
point = QgsPointXY(500000, 4649776)
reprojected_point = transform.transform(point)
print("Reprojected point:", reprojected_point)

🎯 Notes for Students

  • Always check CRS validity with isValid().
  • EPSG codes are the most common and recommended way to define CRS.
  • CRS mismatches are a common source of errors in GIS projects.
  • Understanding CRS ensures accurate distance, area, and spatial analysis.

📚 References