[postgis-users] Invalid geometry? (and side location conflict)

[strk at refractions.net]

On Mon, Nov 06, 2006 at 07:19:38AM -0700, Rob Tester wrote:
> I receive an error when getting the area for an intersection of two geometry objects. One is large containing several hundered points the other was much smaller. 
> 
> I get:
> 
> NOTICE:  TopologyException: side location conflict (-84.4858,33.6872,33.6872)
> 
> ERROR:  GEOS Intersection() threw an error!
> 
> (note that geomunion with the same geometry objects works fine)
> 
> After reading the email lists I saw that invalid geometry objects are often the cause this error, so I checked them with isValid(). Sure enough the large geometry returns false. My question is what really makes a geometry invalid? Is it the invalid geometry that is really causing this issue or something related to the intersection (since geomunion works fine)?

Recent postgis versions should tell you what the invalidity is, through
a NOTICE message.

--strk;