[geos-devel] Converting shape files to GEOS geometries

[strk at refractions.net]

On Wed, Jan 10, 2007 at 02:03:13PM +0100, Jean-Claude Repetto wrote:
> Hello list,
> 
> This is a newbie question :
> 
> I want to create GEOS geometries from data stored in ESRI shapefiles.
> Shapes are composed of several shells and holes. Shells points are
> clockwise, holes are counterclockwise.
> 
> I am using the GEOS C API, as recommended in the documentation at
> <http://geos.refractions.net/ro/doxygen_docs/html/>.
> 
> For each part of the shapes, I have successfully created a CoordSeq,
> then a LinearRing. But to create polygons (before unioning them), I need
> to know :
> - if a ring is a shell or a hole. In the C++ API, there is a function
> called IsCCW. But how can I do the test with the C API ?

You can't (w/out a feature addition to it).

> - what hole belongs to what shell. Is there a simplest method than
> creating polygons for each hole and testing if they are within every
> shell ? Or is it preferable to make an union of all the shells, then
> remove the intersections with the holes ? (Maybe the second method
> doesn't work if there are shells inside a hole).

Try with calling polygonize and then:

        /*
         * Iteratively invoke symdifference on outer rings
         * as suggested by Carl Anderson:
         * postgis-devel/2005-December/001805.html
         */

> Perhaps there is already a program or a library that can do that ?

PostGIS does it:

	o See lwgeom/lwgeom_geos_c.c for using the GEOS CAPI
	  (doesn't need CCW testing)

	o See loader/shp2pgsql.c for getting an WKB or WKT
	  representation you can feed to GEOS.

--strk;