I suspect that Bjorn's shapefile has some features with null geometries. Schuyler's original one did and I removed 5 or 6 of them when packaging it to go with PrimaGIS.
Other caveats about the not-ready-for-prime-time cascaded_union: GEOS fails on invalid geometries such as "bowtie" polygons. These are easy to create in a GIS, but rather expensive to check, too expensive to test .is_valid for every item in a batch. Every geometry passed in to cascaded_union should be valid. Expect an exception if even a single one is not. GEOSUnionCascaded might only work on multipolygons, not on other collections. Your watershed data isn't a collection of multipolygons, is it? If so, you might need to explode them and pass a sequence of polgyons to cascaded_union. Thanks for trying it out, Dane. I'm happy to help you use it. Cheers, Sean On Jan 27, 2009, at 10:36 PM, Dane Springmeyer wrote: > Hey Sean, > > I just rebuilt geos from Paul's 3.1.0rc1.tar.bz2 and was excited to > play around with the cascaded union with a big watershed dataset > that's always been a bear. > > I am on mac 10.5.6 and used your script from: > http://sgillies.net/blog/870/a-more-perfect-union-continued > > It seems to hit the last feature and then crashes python: > > [....] > cxx type MultiPolygon > Assertion failed: (!"should never be reached"), function itemsTree, > file AbstractSTRtree.cpp, line 358. > Abort trap > > Then I tried on Bjorn's border shapefile (v3) (TM_WORLD_BORDERS-0.3) > and also on the last feature it fails, but with this exception: > > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > File "/Library/Python/2.5/site-packages/Shapely-1.1.0-py2.5.egg/ > shapely/ops.py", line 48, in cascaded_union > return geom_factory(lgeos.GEOSUnionCascaded(collection)) > File "/Library/Python/2.5/site-packages/Shapely-1.1.0-py2.5.egg/ > shapely/geometry/base.py", line 35, in geom_factory > raise ValueError, "No Shapely geometry can be created from this > null value" > ValueError: No Shapely geometry can be created from this null value > > Seems odd huh? Let me know if you have any ideas for further testing. > > Thanks! > > Dane > > > On Jan 26, 2009, at 11:11 AM, Sean Gillies wrote: > >> So far so good. >> >> http://sgillies.net/blog/869/efficient-batch-operations-for-shapely >> >> Anyone want to join me as a downstream user and tester of GEOS 3.1? >> >> -- >> Sean Gillies >> [email protected] >> http://sgillies.net >> >> _______________________________________________ >> Community mailing list >> [email protected] >> http://lists.gispython.org/mailman/listinfo/community > > _______________________________________________ > Community mailing list > [email protected] > http://lists.gispython.org/mailman/listinfo/community -- Sean Gillies [email protected] http://sgillies.net _______________________________________________ Community mailing list [email protected] http://lists.gispython.org/mailman/listinfo/community
