[gdal-dev] Topological Union?

Craig Miller craig.miller at spatialminds.com
Fri Aug 29 12:56:37 EDT 2008

Thanks Brent.  I'm looking for a C/C++ solution.  I'll take a look at the other polygon processing libraries as well as the level of effort in porting JTS Cascaded Unions to Geos... that's a conversation for a different mailing list though.


-----Original Message-----
From: Brent Fraser [mailto:bfraser at geoanalytic.com] 
Sent: Friday, August 29, 2008 7:53 AM
To: Craig Miller
Cc: gdal-dev at lists.osgeo.org
Subject: Re: [gdal-dev] Topological Union?


  I've had limited success with the Union function in Geos (via GDAL).  It's slow and fails on large polygon sets (some of my files have 10,000 polygons with some convoluted geometry), but if your sets are smaller, it may work.  FYI, Geos is derived (ported?) from JTS; Martin Davis has a good explanation of union strategies at http://lin-ear-th-inking.blogspot.com/2007/11/fast-polygon-merging-in-jts-using.html.  Currently, I'm considering using Geotools (http://geotools.codehaus.org/) as a framework to get at the JTS functionality.

  And there are other polygon processing libraries out there.  There's a nice summary at http://www.complex-a5.ru/polyboolean/comp.html, but I haven't tried any of those yet.

Good Luck!
Brent Fraser

Mateusz Loskot wrote:
> Craig Miller wrote:
>> Thanks Mateusz.  That's exactly what I was thinking of doing if
>> someone hasn't put it together already.
> Craig,
> BTW, you don't have to convert OGRGeometry (you play with it when 
> reading Shapefile using OGR) to geos::geom::Geometry.
> You can union OGRGeometry using OGRGeometry::Union()
> or its C API equivalent OGR_G_Union
> These operations are based on GEOSUnion function.
> Best regards,

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