CRS (Coordinate Reference System)
A definition of how coordinates map to real locations on Earth, including projection, datum, and units.
Category: CRS & Coordinates
Definition (expanded)
CRS tells you what the numbers mean. EPSG:4326 is lon/lat degrees. Web Mercator (EPSG:3857) is a projected CRS used for many web maps. If you buffer or measure, CRS matters because doing meters-based operations on degrees can be misleading.
Related terms
EPSG:4326 (WGS84 lon/lat)A common coordinate reference system using longitude/latitude on WGS84.Web Mercator (EPSG:3857)A common projected CRS used by web maps; it distorts area and shape but is fast and convenient for tiles.ProjectionA mathematical method for mapping the curved Earth to a flat plane, which affects distance, area, and shape.GeodesicMeasurements or operations computed on the curved Earth model rather than a flat plane.UTM (Universal Transverse Mercator)A projected coordinate system split into zones, commonly used for accurate local measurements in meters.Datum (geodetic datum)The Earth model a CRS is based on; different datums can shift coordinates even if units look the same.EPSG codeA standard numeric identifier for a coordinate reference system (e.g., 4326 for WGS84 lon/lat).SRIDA numeric identifier for a coordinate reference system used by some databases and formats (especially PostGIS and EWKB).Reprojection (coordinate transformation)Converting coordinates from one CRS to another (e.g., EPSG:4326 lon/lat → EPSG:3857 meters).