[postgis-users] Semantics of difference() function

Martin Davis mbdavis at refractions.net
Wed Jun 4 09:55:50 PDT 2008

Ah, right - now I see why you're getting a multipolyon back, due to the 
new vertices being introduced.  Welcome to the world of finite-precision 
geometry!  It's kind of like quantum physics - ordinary geometric 
operations work differently and sometimes have peculiar side effects 
down at the vertex scale....

Obe, Regina wrote:
> Actually the PostGIS/GEOS behavior is consistent with your JTS I 
> think.  What I was getting was a single Polygon too with extra vertices
> where the line cut throw.  It was when I was trying to figure out what 
> the difference was between my original
> polygon and this new polgon difference line.  Simply overlaying them 
> on a screen I couldn't tell the difference so I differenced them and 
> was shocked when my old polygon was completely contained by new 
> polygon and new polygon was bigger by 2 super small polygons.  I 
> thought it was more funny than anything.

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

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