[gdal-dev] Calculate footprints of shapefiles

[Armin Burger]

Chaitanya

I can understand how the UnionCascade works in PostDIS since it's an aggregate function. I just don't understand how I should use it in OGR. The documentation has a single line 
    UnionCascaded(self) -> Geometry

So it returns a geometry from another geometry, but I would need a returned geometry from a *list* of input geometries, not from a single one. So I don't have a clue if this is possible in OGR. 

Another option could be the "Collect" function, but this seems to be only implemented in the C libraries, not in the Python bindings. Looks like geometry collections are not available in OGR for Python at all.

Armin


-------- Original-Nachricht --------
> Datum: Thu, 17 Mar 2011 10:16:06 +0530
> Von: Chaitanya kumar CH <chaitanya.ch at gmail.com>
> An: Armin Burger <armin.burger at gmx.net>
> CC: gdal-dev at lists.osgeo.org, mariusjigmond at hotmail.com
> Betreff: Re: [gdal-dev] Calculate footprints of shapefiles

> Armin,
> 
> Cascaded Union works pretty much like Union except that it is optimized to
> work on more than two geometries.
> Both OGR and PostGIS uses the GEOS library to perform this.
> 
> Read this blog entry by Paul Ramsey:
> http://blog.cleverelephant.ca/2009/01/must-faster-unions-in-postgis-14.html
> 
> On Wed, Mar 16, 2011 at 3:41 PM, Armin Burger <armin.burger at gmx.net>
> wrote:
> 
> > Chaitanya
> >
> > thanks for the hints. So a sort of Union operation seems to be
> necessary. I
> > need to check how fast that is with OGR on larger shapefiles (I want to
> > store in PostGIS just the footprint, not the full geometry collection).
> The
> > ConvexHull could be a good alternative to a simplified boundary.
> >
> > I understand how the normal Union() works, it merges 2 geometries into a
> > new one. But I don't get it what UnionCascade() is doing. It applies a
> Union
> > on a single geometry and returns the new geometry, what is it actually
> > unioning/merging then?
> >
> > Regards,
> > Armin

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