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 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.
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
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--
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341
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