[postgis-users] Re: Comparing Geometries with Different SRIDs

[Reid Priedhorsky]

Charlie Savage wrote:
>> The 2 point could be spatially equal given different SRIDS and 
>> coordinates if they were projected to a common SRID. So should
>> geometry operators silently Call st_transform to make the righthand
>> match the lefthand before comparing? This would be quite the
>> expensive operation.
> 
> No. You can't automatically transform between different SRID values 
> Think of the case of one geometry with an SRID value of 4326 and one
>  with an SRID value of -1 (no coordinate system).

Exactly.

> So different SRID values, then the geometries are not equal.

No -- as Stanley said, the geometries could be in fact equal, but 
expressed in different SRS. So if ST_Equals() returned False, it would 
be wrong.

> It is up to the user to transform geometries to the same SRID before
> calling ST_EQUALS.

Exactly. ;)

Reid