[postgis-users] PostGIS 1.3.3 on Windows XP Unstable

Martin Davis mbdavis at refractions.net
Wed Aug 13 10:10:58 PDT 2008

Bresnahan, Mike wrote:
> I'm almost there. The function works beautifully except on a few
> geometries. These geometries produce messages like the following when I
> call ST_IsValid:
> NOTICE:  Nested shells at or near point -76.554 38.8815
> I'm not sure what "nested shell" means. At first thought it says to me
> that one external polygon is contained inside another external polygon,
> but isn't that legal?
That's what it means, alright.  It is invalid to have one polygon inside 
another one in the same MultiPolygon.  (If you post the invalid geometry 
I can confirm that this is what is happening).

Bottom line is that you need to check for two things:
- holes which have "flipped" to be outside their parent shell
- polygons which have flipped to be inside another polygon

Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022

More information about the postgis-users mailing list