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

[Paul Ramsey]

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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>>> postgis-users at postgis.refractions.net
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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