[postgis-users] Polygon validity

[Martin Davis]

This polygon is valid according to the OGC SFS, and thus validates 
correctly in PostGIS (also GEOS/JTS).

You'd have to look at the validation semantics of the other systems to 
determine why they don't validate it.  The OGC SFS model is fairly 
tolerant in terms of trivial structural aspects - for instance, it 
allows duplicate points.  I believe this is done in order to allow as a 
wide a range of geometries as possible to be represented, consistent 
with topological integrity.  Some other systems are less tolerant.

So I think strk is probably right - perhaps they don't allow duplicate 
points, or perhaps they are reducing the precision and thus causing a 
self-intersection (although I tried this in JTS, and reducing precision 
doesn't actually cause self-intersections)

Martin

Rykov Denis wrote:
> There is following polygon:
> POLYGON((317371.62563763 5731575.49932608,
> 317028.110300846 5731043.43103426,
> 317028.110300846 5731043.43103426,
> 316389.822682815 5731138.79331604,
> 316478.209591618 5731705.00321977,
> 316478.209591619 5731705.00321977,
> 317350.223258488 5731584.46661525,
> 317371.62563763 5731575.49932608))
>
> with two duplicate nodes. When I try to check it with ST_IsValidReason
> function I get message
> that it is Valid Geometry. Is this OGC compliant? When I try to check
> it with QGIS, ArcView or
> ArcGIS - I get notice about self intersection. Please explain this situation.
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-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022