Martin Davis mbdavis at refractions.net
Wed Mar 19 18:21:47 EDT 2008

```Chris has got it - the GEOS simplify won't do what you want, since it
only simplies a single feature.  In order to do this, you would need to
build a single-line coverage, simplify the edges of it, and then rebuild

Chris Hodgson wrote:
> I think to do this correctly you need to have the concept of a
> coverage, and the simplification algorithm needs to work on the
> coverage. As it simplifies each edge in the coverage, it would need to
> be aware that the edge is part of two faces, and check that it has to
> maintain the validity of both polygons as it simplifies. I think this
> would be a pretty significant development task.
>
> If your polygons aren't too tricky, you might be able to take a
> simpler approach - intersect every polygon with it's neighbors and
> create a new dataset of every line, with 2 additional attributes
> specifying the IDs of the two polygons (faces) that shared that line
> (edge). Then simplify each of these edge lines, and then rebuild your
> polygons from the simplified edge lines. The problem being, there is
> no guarantee that your polygon's topologies haven't been broken by the
> simplification.
>
> I don't think your multi-geometry approach will work because I don't
> believe that GEOS will treat the geometries inside the multi-geometry
> any differently than if they were simplified separately - as you are
> already doing. I haven't tried it or read the docs so I may be wrong.
>
> HTH.
>
> Chris
>
> Bill Zissimopoulos wrote:
>> Hello, list:
>>
>> I am looking for a method to correctly simplify adjacent polygons
>> without leaving gaps between them or having them intersect after the
>> simplification. Currently my simple algorithm iterates over my
>> geometries and feeds them to GEOSTopologyPreserveSimplify separately;
>>
>> Is it possible to correctly simplify adjacent polygons in GEOS? One
>> technique I have been thinking about is to construct a multi-geometry
>> with all the geometries in my dataset and pass this multi-geometry to
>> GEOSTopologySimplify. Even if this worked, I would have to identify the
>> individual geometries in the resulting multi-geometry since they
>> represent different features in the original dataset. For example, a
>> lake polygon and a land polygon next to it would have to be identified
>> again after simplification so they can be properly rendered.
>>
>> The solution here might be to abuse slightly the z coordinate to store
>> tagging information about individual polygons. But I wanted to hear any
>> comments from the experts on the list before I attempt this.
>>
>>
>> Bill
>> _______________________________________________
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>> geos-devel at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/geos-devel
>
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>

--
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022

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