This is a known bug with the SimplifyPreserveTopology in both JTS and
GEOS. Hopefully it will be fixed soon. It's not a difficult fix, but
it requires some resources (funding) to git 'er done.
[email protected] wrote:
Hi,
I have just created a multipolygon column & populated it with a
ST_SimplifyPreserveTopology SQL.
I checked the resulting geometries with isvalid() and found some errors.
I fixed these by editing the geometries with QGIS. The errors were small
islands outside the major polygons in the original multipolygon, or small holes
near the edge. When the geometry was simplified, these small polygons finished
up as holes outside the polygon, or islands inside it, which I could find using
isvalid(). I'm assuming any topological error found by isvalid() in a geometry
produced by ST_SimplifyPreserveTopology() indicates a bug in
ST_SimplifyPreserveTopology().
I exported the now validated column to shapefile with pgsql2shp which worked
fine, but ogr2ogr failed to process this shapefile, with several errors of:
ERROR 1: Corrupted .shp file : shape 17 : panPartStart[0] = 0, nVertices = 0
I can skip these errors, but I'd hoped that ST_SimplifyPreserveTopology() &
isvalid() would ensure my simplified geometries were free of such errors.
Ideally PostGIS should not generate such things, and isvalid() should identify
such errors.
If it helps, the geometries are US ZCTA 5 digit polygons, & the version of
PostGIS I'm using is:
postgis_full_version
-------------------------------------------------------------------------------------
POSTGIS="1.3.5" GEOS="3.0.2-CAPI-1.4.2" PROJ="Rel. 4.6.1, 21 August 2008"
USE_STATS
Thanks,
Brent Wood
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--
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022
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