9.34.1.Spatial References
The default reference system is WGS-84 with coordinates in
degrees of longitude and latitude on Earth. The SRID number for
this is 4326 and is assigned by default to all new geometries.
ST_Transform
can
be used to re-calculate coordinates of a shape from one spatial
reference system to another. ST_SetSRID
can be used for altering SRID
without altering coordinates.
Distances and precisions for "latitude-longitude" spatial reference systems are in kilometers, as approximately calculated with the haversine formula. For other systems they are in the same unit as the coordinates.