On Tuesday 12 February 2008, Paul Ramsey wrote: > On 12-Feb-08, at 10:30 AM, Dylan Beaudette wrote: > > Am I correct in understanding that the ST_SimplifyPreserveTopology > > () function > > would give topologically correct polygons after simplification? In > > other > > words, it would preserve adjacency after simplification? > > No, incorrect, perhaps this calls for a renaming. It will preserve > validity, that is, each individual polygon will remain topologically > correct.
I see-- thanks for the clarification. Perhaps a rename would be a good idea, maybe ST_RobustSimplify ?? > > I have noticed that running simplify() on a set of adjacent > > polygons sometimes > > has the not-so-nice result of causing previously overlapping (dang > > simple > > features!) edges to separate. > > Sorry, that remains a hard problem to crack. I doubt anything short > of a build-coverage/simplify-edges/extract-polygons approach is going > to do it. > > P. Ok. I primarily use the simplify function for display at very coarse scales -- where these little "nuances" are not visible. Thanks, Dylan > > Dylan > > > >> On Feb 8, 2008, at 10:21 PM, Brian Hamlin wrote: > >>> It recently came up in the IRC channel FreeNode #postgis > >>> that there is a GEOS TopologyPreserveSimplify, in addition > >>> to Simplify2D. > >>> > >>> I did a bit of checking just now.. on my machine, I see > >>> nm /usr/local/pgsql/lib/liblwgeom.1.3.so > >>> > >>> _LWGEOM_simplify2d > >>> _pg_finfo_LWGEOM_simplify2d > >>> _simplify2d_lwgeom > >>> > >>> and some others, but no TopologyPreserve anything > >>> > >>> nm /usr/lcoal/lib/libgeos_c.1.4.1.dylib > >>> > >>> _GEOSSimplify > >>> _GEOSTopologyPreserveSimplify > >>> > >>> and lastly, in lwpostgis.sql > >>> CREATE OR REPLACE FUNCTION simplify(geometry, float8) > >>> RETURNS geometry > >>> AS '$libdir/liblwgeom', 'LWGEOM_simplify2d' > >>> LANGUAGE 'C' IMMUTABLE STRICT; > >>> > >>> -- > >>> since the last SQL line matches the exported symbol name from > >>> liblwgeom > >>> exactly, minus the front _, looks like there is the exact entry > >>> point. > >>> > >>> Clearly no parallel entry for TopologyPreseveSimplify > >>> > >>> this makes me think that there is a little more to adding the > >>> call to > >>> lwpostgis.sql than copying and pasting the declaration. > >>> > >>> insights welcome > >>> -Brian > >>> > >>> _______________________________________________ > >>> postgis-users mailing list > >>> [email protected] > >>> http://postgis.refractions.net/mailman/listinfo/postgis-users > >> > >> _______________________________________________ > >> postgis-users mailing list > >> [email protected] > >> http://postgis.refractions.net/mailman/listinfo/postgis-users > > > > -- > > Dylan Beaudette > > Soil Resource Laboratory > > http://casoilresource.lawr.ucdavis.edu/ > > University of California at Davis > > 530.754.7341 -- Dylan Beaudette Soil Resource Laboratory http://casoilresource.lawr.ucdavis.edu/ University of California at Davis 530.754.7341 _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
