[gdal-dev] negative buffer

Clay, Bruce bclay at ball.com
Tue Sep 9 11:13:56 EDT 2008

I am trying to use GDAL to cluster a group of points.  I am creating a
buffer around the points then testing buffered points for intersection.
I am putting those that intersect into a GeometryCollection then
creating a convexHull to smooth the polygon.


All of that works ok but the problem is that the resultant polygon is
larger than is desired because of the buffer used to create the


I tried to use a negative value with the Geometry Buffer method to
shrink the polygon back down to the proper size but Buffer always
returns a NULL pointer.  I stepped down into the call and it looks like
GEOS is returning a valid geometry but OGR rejects the polygon do to the
number of points being too low.  The initial buffer size is 1.5 Km so
smallest cluster has a minimum spatial diameter of 3 km.  I saw point
counts in the debugger of over 2000 so I am not sure why OGR appears to
reject the geometry that GEOS passed back.  I don't have GEOS compiled
with debug turned on so I could not step into that


Is there another way to reduce the size of a polygon or a smarter war to
cluster points?






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