Coordinate Reference System (CRS)

Overview A Coordinate Reference System (CRS) provides the full mathematical and physical framework needed to assign coordinates to locations on or near the Earth's surface. A CRS combines a datumDatumA geodetic datum is a mathematical model that defines the size, shape, and orientation of the Earth, serving as the r... (which models the Earth's shape and anchors the system to physical reality), a coordinate system (which defines the axes and units), and for projected systems, a map projectionMap ProjectionMap projections are mathematical transformations that convert the three-dimensional surface of the Earth onto a two-d... (which flattens the curved surface onto a plane). Understanding CRS is essential for ensuring that geospatial dataGeospatial DataGeospatial data encompasses information about the location, shape, and relationships of physical features on Earth. I... aligns correctly and measurements are accurate.

Geographic CRS use angular coordinates (latitude and longitude) referenced to an ellipsoid, such as WGS 84WGS 84WGS 84 (World Geodetic System 1984) is the global geodetic reference system used by GPS and most modern mapping appli... (EPSG:4326) or NAD 83NAD 83NAD 83 (North American Datum of 1983) is the horizontal geodetic datum used for surveying, mapping, and navigation ac... (EPSG:4269). Projected CRS apply a map projectionMap ProjectionMap projections are mathematical transformations that convert the three-dimensional surface of the Earth onto a two-d... to convert angular coordinates into planar coordinates measured in linear units like meters, such as UTM Zone 33N (EPSG:32633) or British National Grid (EPSG:27700). Vertical CRS define height or depth relative to a reference surface such as mean sea level. Compound CRS combine horizontal and vertical systems for three-dimensional positioning.

Every spatial dataset has an associated CRS that defines the meaning of its coordinate values. When combining datasets with different CRS, GISGISGeographic Information Systems (GIS) enable users to analyze and visualize spatial data to uncover patterns, relation... software performs coordinate transformations to align them in a common reference frame. Incorrect CRS assignment or missing CRS metadata is one of the most common sources of error in GIS projects, potentially causing features to appear in wrong locations or measurements to be inaccurate. Modern GIS platforms support on-the-fly reprojection, transparently transforming data for display without altering the stored coordinates.

Always verify the CRS of incoming data before use. Store data in the CRS most appropriate for its geographic extent and intended analysis. Use authoritative EPSG codesEPSG CodesEPSG codes are standardized numeric identifiers assigned to coordinate reference systems, datums, and map projections... to specify CRS unambiguously. When performing area or distance calculations, use a projected CRS with appropriate units rather than geographic coordinates to ensure accurate results.