srid

SRID stands for Spatial Reference System Identifier. It is a numeric code used by geographic information systems (GIS) and spatial databases to reference a specific coordinate reference system (CRS). An SRID does not store coordinates itself; it points to the formal definition of how coordinates are measured, interpreted, and transformed within a given CRS.

A CRS defines how positions on the Earth are expressed, either as geographic coordinates in angular units (degrees) with a datum, or as projected coordinates in linear units (meters, feet) using a map projection. The full meaning of a coordinate pair depends on the CRS, and transforming coordinates between CRSs may require a datum shift or a projection change.

CRS definitions are maintained in registries, with the EPSG Geodetic Parameter Dataset being the most widely used standard. EPSG assigns codes such as 4326 for WGS 84 geographic coordinates and 3857 for Web Mercator. Other common SRIDs include 27700 (British National Grid, OSGB36) and 32633 (WGS 84 / UTM zone 33N). In practice, SRID is used to attach a geometry to a CRS in a database or GIS, and to perform transformations between CRSs, for example using functions that set or transform the SRID of geometries.

Common pitfalls include using data with incompatible datums or units without proper transformation, which can lead to spatial misalignment. It is important to verify the CRS associated with each dataset and apply appropriate coordinate operations when combining data from different SRIDs.

See also: coordinate reference system, CRS registry, EPSG, WGS 84, map projection.