[postgis-users] Semantics of difference() function
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.
Senior Technical Architect
Refractions Research, Inc.
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