[postgis-users] st_simplifypreservetopology

Mon Apr 21 04:48:08 PDT 2008

Hi,

i am not sure whether i should point this issue to the postgis developer forum or maybe even to the GEOS developers:

my postgis version is:
select postgis_full_version()
POSTGIS="1.3.3" GEOS="3.0.0-CAPI-1.4.1" PROJ="Rel. 4.6.0, 21 Dec 2007" USE_STATS

and i encounter the following problem
the new function of postgis 1.3.3 st_simplifypreservetopology does not seem to work as specified in the postgis reference. the reference says that "ST_SimplifyPreserveTopology(geometry, tolerance) Returns a 'simplified' version of the given geometry using the Douglas-Peuker algorithm. Will avoid creating derived geometries (polygons in particular) that are *invalid*". but in fact st_simplifypreservetopology does not seem to prevent returning invalid geometries. i have just tested it with the statement below. i will attach the geometry source for testing purposes.

it would be nice if ST_SimplifyPreserveTopology would work in the specified manner, because it would enable postgis user to do a st_intersection(geometry,geometry) on simplified (and then still valid) data without getting the annoying geos error: 
"NOTICE:  TopologyException: side location conflict 14.609 51.552
ERROR: GEOS Intersection() threw an error!
SQL Status:XX000"

maybe i should have tested this during the 1.3.3 release candidate period.

any ideas?

best regards,
stefan

select st_astext(_g2870."the_geom"), st_astext(st_simplifypreservetopology(_g2870."the_geom",0.0057)), st_isvalid(_g2870."the_geom"), st_isvalid(st_simplifypreservetopology(_g2870."the_geom",0.0057))
FROM _g2870

st_astext,	st_astext,	st_isvalid,	st_isvalid
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51.0473564,13.7390821 51.047287,13.7390933 51.047209,13.7390933 51.0471421,13.7368533 51.0470386)),((13.7367293 51.0470057,13.7346615 51.0466892,13.7347551 51.0468411,13.7347754 51.0470359,13.7347106 51.0471899,13.7356421 51.0472919,13.7366963 51.0474074,13.736705 51.047249,13.7367293 51.0470057))),
MULTIPOLYGON(((13.73095 51.024734,13.7123153 51.041449,13.7552745 51.0566237,13.7484397 51.0324582,13.73095 51.024734)),((13.7368533 51.0470386,13.7369099 51.0474154,13.7390933 51.047209,13.7390933 51.0471421,13.7368533 51.0470386)),((13.7367293 51.0470057,13.7346615 51.0466892,13.7347106 51.0471899,13.7367293 51.0470057))),	
true,
false
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