I wonder why the Authority and the SRID are often both used together to refer to a CRS, like in
Isn't the SRID unique?
Wikipedia says it is a "unique value", but also that spatial vendors may have created "[...] their own SRID implementation or refer to those of an authority, [...]". What does that mean, creating an own SRID implementation?
After browsing through spatialreference.org it seems that there are no duplicate SRIDs for different authorities.
If this is true and SRIDs are truly unique, why the OGC
spatial_ref_sys metadata table (see Wikipedia link above; you may find this table in databases like PostGIS) is defined with both, an
SRID column as well as an
AUTH_SRID column? In a default PostGIS setup (6184 EPSG and 2315 ESRI entries), both columns always have the same value per row. If I wanted to add an user-defined entry, why would I want to choose different values for those columns?
Or if I were an authority, what could be the rationale for a decision that would cause me to define an SRID that has already been defined by another authority?