[postgis-users] Re: Difference throws an error

Martin Davis mbdavis at refractions.net
Tue Nov 7 14:29:00 PST 2006

The reason the error is thrown is that at least one of the geometries 
has a very gnarly spike on one side of it.  The second geometry contains 
the vertices:

[      0:28] POINT (5417154.92 5658376.37)
[      0:29] POINT (5417155.18955743 5658376.89699992)
[      0:30] POINT (5417154.919 5658376.371)
[      0:31] POINT (5417155.814 5658378.111)

These form a spike/gore where the inside vertex 29 is almost coincident 
with the edge 30-31.

This geometry combination causes an error in all previous versions of 
JTS/GEOS as well.  Hopefully this problem can be addressed with some 
further work on robustness.

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

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