Reprojection (coordinate transformation)

Converting coordinates from one CRS to another (e.g., EPSG:4326 lon/lat → EPSG:3857 meters).

Category: CRS & Coordinates · Also known as: reproject, coordinate transformation, ST_Transform, transform CRS

Definition (expanded)

Reprojection changes the coordinate numbers to match a different coordinate reference system. It’s essential when combining datasets from different CRSs or when you need meters for distance/area operations. Always confirm axis order and the source CRS before transforming.

Examples

PostGIS reprojection (4326 → 3857)

SQL

SELECT ST_AsText(ST_Transform(ST_SetSRID(ST_GeomFromText('POINT(12 55)'), 4326), 3857));

Common mistakes

Transforming coordinates without setting/knowing the source CRS first.
Assuming Web Mercator is suitable for accurate measurements everywhere.

Related terms

CRS (Coordinate Reference System)A definition of how coordinates map to real locations on Earth, including projection, datum, and units.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).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.Axis orderWhich axis comes first for coordinates, commonly x then y, but some services and standards use latitude first.Datum (geodetic datum)The Earth model a CRS is based on; different datums can shift coordinates even if units look the same.