[postgis-users] beginner questions re. SRID

[Markus Schaber]

Hi, Paul and Rick,

Paul Ramsey wrote:

>> Is there a more human-readable table that contains the SRIDs and 
>> descriptions?
> 
> More than spatial_ref_sys? Not really. The SRTEXT isn't completely 
> *un*readable though.

It may be that the EPSG database (where the spatial_ref_sys is derived
from) contains some more verbose descriptions, but I doubt that it will
be of much more use.

> Yes! Confusing! But part of the standard.  The specification says  that
> all geometries have an SRID.  It also says that the  geometry_columns
> table contains a single SRID for each spatial column.

Note, however, that in PostGIS you can avoid this constraint and create
geometry columns that contain mixed-srid data, or even geometries
without any specified SRID (represented as -1). PostGIS will work fine,
but you're not OpenGIS standard compliant any more, and index usage may
suffer.

>> Can it properly compare an object with one SRID with an object with  a
>> different SRID?
> 
> No, and most/many functions will error out if you try, because it is 
> not really correct to compare geometries with different SRID (unless 
> they happen to map to the same projection! ha!).

One can use something like

        intersection(geoma, transform(geomb, srid(geoma)))

to compare geometries which (potentially) have different SRIDs.

It works fine, but may be rather slow depending on your exact data and
query.


Markus