SRIDs

SRID stands for Spatial Reference System Identifier. It is a numeric code that uniquely identifies a coordinate reference system (CRS) used to locate geographic features on the Earth. Each SRID corresponds to a defined CRS that specifies how coordinates map to locations, including the datum, projection, and units. Spatial data formats and databases attach an SRID to coordinates so they can be interpreted and combined with other data.

The most commonly used registry of SRIDs is the EPSG dataset, maintained by the International Association of Oil & Gas Producers (IOGP). It defines thousands of geographic and projected coordinate reference systems, each assigned a unique integer code. In software, that code is usually stored as the SRID value and used to drive coordinate interpretation and transformations. Vendors may also define separate aliases or namespaces, such as Esri’s codes, which can be cross-referenced to EPSG.

Common examples include EPSG 4326 for WGS 84 geographic coordinates (latitude and longitude in degrees) and EPSG 3857 for Web Mercator. Projection-based systems include codes like 32633, which represents WGS 84 / UTM zone 33N, and 32733 for zone 33S. There are many more codes covering global, regional, and local CRS definitions.

In practice, SRIDs enable coordinate transformations and reprojection in GIS workflows. Functions like ST_Transform reproject data from a source SRID to a target SRID. If the SRID attached to data is missing or inconsistent with the intended CRS, analyses and map outputs can be incorrect, underscoring the need for consistent CRS management.