[postgis-users] why is this poly invalid?

Martin Davis mbdavis at refractions.net
Tue Oct 30 09:02:29 PDT 2007

It might be nicer if it worked that way, but currently that's not the 
case.  The justification is:
- geometric functions are only guaranteed to return correct results on 
valid input (except for isValid, of course)
- valid polygonal geometries are by definition simple

In other words, the domain of isSimple is really just linear geometry - 
there's no point in calling it for polygonal geometry.

Rob Agar wrote:
> Nicolas Ribot wrote:
>> No, self-intersecting polygons are not valid in postgis.
> OK, that makes sense.  But shouldn't st_issimple return false for a 
> self intersecting poly?
> > Try to edit it to correct the intersection.
> I will - it's just a matter of swapping a couple of points. But I'll 
> tell the Great Barrier Reef Marine Park Authority that their data's 
> dodgy :)
> Rob
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Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022

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