[postgis-devel] polygonize_garray build invalid geometries

[strk at refractions.net]

The problem here is in the implementation of the postgis polygonize
interface. GEOS polygonize returns a list of polygons, not a
multipolygon. Postgis is *forcing* single-geometry output, creating
a multipolygon, which probably often results in an invalid geometry.

I think the fix here is creating a collection, instead of a MultiPolygon.
Note that in this case postgis lacks a polygon construction mean.

I suggest make_polygon(geometry, geometry[]), having shell as first
argument and a list of holes as second (optional) argument.

--strk;

On Sun, Jan 02, 2005 at 12:01:35PM -0500, Carl Anderson wrote:
> against CVS Postigs and CVS GEOS
> 
> polygonize_garray in instances returns invalid geometries
> 
> test cases avail at 
> http://wms.co.fulton.ga.us/data/postgis/polygonize_error.sql.gz
> (sorry, it was too big for the list)
> 
> run the script to build object  then
> 
> select isvalid(the_geom) from polygonize_error;
> 
> select
>   isvalid(polygonize_garray(array_append('{}',
>           intersection(boundary(the_geom),boundary(the_geom))
>        )))
>   from polygonize_error;
> 
> intersection(boundary(),boundary())  -- converts all rings into simple 
> linestrings (actually OGC line geometries  SFSSQL 2.1.6)
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