Spatialrefsys

Spatialrefsys, typically implemented in a PostGIS environment as the spatial_ref_sys table, is a catalog of coordinate reference systems (CRS) used to interpret and transform spatial data. It provides a centralized registry that maps human-readable CRS identifiers to machine-readable definitions, enabling consistent handling of geometry coordinates across databases and applications.

The table stores metadata for each defined CRS. Core columns commonly include a srid (integer, the unique identifier), auth_name (text, the authority that defined the CRS, often EPSG), auth_srid (integer, the authority code), srtext (text, the Well-Known Text representation of the CRS), and proj4text (text, the legacy PROJ.4 string). The srtext field conveys a full geometric description in a portable text format, while proj4text historically offered a compact, parseable projection string for older tools.

Entries are usually sourced from standard authority datasets such as EPSG, but databases may also include custom or locally defined CRS. This catalog is essential for operations that rely on knowing how coordinates relate to real-world locations, including reprojection with ST_Transform, assignment of CRS with ST_SetSRID, and spatial indexing that depends on the CRS for accurate queries.

Users typically query spatial_ref_sys to discover the SRID definitions in use, for example by selecting a record with a specific srid or by listing all available CRS. In practice, the table underpins critical GIS tasks: interpreting geometry, converting between coordinate systems, and ensuring interoperability among datasets.