[gdal-dev] Intersection of polygon layers

[Francis Markham]

If you're using Python, I'd recommend checking out the shapely package (
http://pypi.python.org/pypi/Shapely ):

You can load your shapefile using ogr, and then import the geometry to
shapely where you can do the geometry predicate calculations:

>>> import ogr
>>> from shapely.wkb import loads
>>> source = ogr.Open("/tmp/world_borders.shp")
>>> borders = source.GetLayerByName("world_borders")
>>> feature = borders.GetNextFeature()
>>> loads(feature.GetGeometryRef().ExportToWkb())


Good luck,

Francis

On 6 November 2010 06:08, yosuke kimura <yosukesabai at gmail.com> wrote:

> All,
>
> I am trying to calculate intersection of two polygon layers with
> polygons.  I searched threads, found following thread, and what I got
> was that OGR/GEOS provide me to intersection of two features, but I
> have to write something in order to perform this operation for two
> layers with multiple features.
>
> Open source vector geoprocessing libraries?
> http://groups.google.com/group/gdal/browse_thread/thread/7b5a4f461a4db319/
>
> Intersect of two shapefiles
> http://groups.google.com/group/gdal/browse_thread/thread/4b178c776aecf425/
>
> My questions are:
>
> 1. Did I miss something?  Is there development of library which takes
> two polygon layers to calculate intersection or other basic geometry
> operations that i find in OGR?
>
> 2. Assuming no to above, I got started writing code using python
> binding.  Not having too much experience, I codes to go through all
> possible pair (complete bipartite), test with intersect, then run
> intersection if it returns true.  I am wondering there is faster way
> to do this.  I am thinking something like get envelope polygon of each
> feature first, and then test intersection of them and then perform
> intersect operation only when first test passes (i assume that it is
> easier to test intersect if there are only handful of vertices as
> oppose to hundreds of vertices in my case).
>
> Right now I am trying with layer of hundreds of thousands of polygons
> vs. thousands of polygon, and it seems to be taking 10s of hours to
> finish in my environment.
>
> Thank you,
>
> Yosuke Kimura
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