module documentation

The GeometryColumns and SpatialRefSys models for the PostGIS backend.

Class PostGISGeometryColumns The 'geometry_columns' view from PostGIS. See the PostGIS documentation at Ch. 4.3.2.
Class PostGISSpatialRefSys The 'spatial_ref_sys' table from PostGIS. See the PostGIS documentation at Ch. 4.2.1.