How to build a multipolygon from a list of rings?

[Liangxiong Zhu]

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Hi there,

 

I have a list of rings which are read from shapefile.  all out rings are clockwise and all hole rings are counterclockwise. After  reading  the OGC spec, it seems that i have to decide for each hole ring that to which out ring it belongs. Which means i have to build a list of Polygon first and then build a MultiPolygon.

 

how could i build a MultiPolygon from a list of rings  directly? It is hard for me to find out the relationship of holes and out rings.

 

Regards,

Abe Zhu

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[Frederik Ramm]

Hi,

[hidden email] wrote:
> how could i build a MultiPolygon from a list of rings  directly?

Have you tried using the Polygonizer class to create a collection of
polygons from your data, then make a multipolygon from that?

> It is
> hard for me to find out the relationship of holes and out rings.

I had a similar situation once and cursed GEOS for not allowing me to
find out which ring is inside another - I first had to convert all rings
to polygons individually, then I could check each of these polygons
(using the "contains" method) whether it was contained in any of the
others, and this then allowed me to construct a proper createPolygon
call (or multiple of those).

Bye
Frederik
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[strk]

Consider looking at shp2pgsql.c from PostGIS project.
It does contain code to build SFS geometries out of a set of rings.

--strk;

On Fri, Oct 09, 2009 at 02:51:55PM +0800, [hidden email] wrote:

> Hi there,
>
> I have a list of rings which are read from shapefile.  all out rings are clockwise and all hole rings are counterclockwise. After  reading  the OGC spec, it seems that i have to decide for each hole ring that to which out ring it belongs. Which means i have to build a list of Polygon first and then build a MultiPolygon.
>
> how could i build a MultiPolygon from a list of rings  directly? It is hard for me to find out the relationship of holes and out rings.
>
> Regards,
> Abe Zhu
> _______________________________________________
> geos-devel mailing list
> [hidden email]
> http://lists.osgeo.org/mailman/listinfo/geos-devel

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