[postgis-users] Adding a new GEOS call to PostGIS

[Dylan Beaudette]

On Tuesday 12 February 2008, Paul Ramsey wrote:
> OK, it was a bit harder than you interpreted (takes more than a SQL
> cut'n'paste) and not as straightforward as I thought it might be (the
> simplify function that was already there was a native implementation,
> not a GEOS call, so cutting and pasting it would do no good).
>
> Anyhow, I have applied my incredible text editing skills and now there
> is a ST_SimplifyPreserveTopology(geometry, float) in the -svn drop.
>
> Paul

Nice work Paul-

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?

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. 

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