[geos-devel] how to construct geos geometry instances using python bindings

Sean Gillies sgillies at frii.com
Mon Mar 5 11:42:22 EST 2007

N.J. Hardebol wrote:
> Hello,
> I'm a medium level ocassional developper with not too much experience with
> python classes and especially not swig. I use python scripting a lot for
> medium level scripting for processing of geographic oriented data. Thereto
> I make use of gdal.ogr and geos libraries ported to python. Previously I
> ran my code under linux, allowing to build from newest source codes.
> Recently I moved code also to windows to interact also with ArcGIS, so
> limitted by available bin + swig compilations.
> I've two questions, both to do with the construction of new geos geometry
> instances.
> In first problem, I start with WKT or WKB geometry strings retrieved from
> postgis database. Afterwards I want to parse them to geos Geometry
> instances.
>>>> geosgeom = geos.WKTReader(geos.GeometryFactory()).read(wktgeom)
> This looks really awkward and as fact of the matter it doesn't build a
> geos.Geometry but a geos.GeometryPtr.
> So my Question1 is: How do i best build a geos Geometry from WKT-strings
> using geos-python bindings?
> Then I also would like to crop an existing geos Linestring1 with an
> intersecting geos Polygon building a new geos Linestring2. So something
> like  Linestring2 = Linestring1.crop(Polygon). My workaround is (1) check
> whether intersect exists. (2) Find the intersection points between
> Linestring1 and Polygon. (3) Build the new linestring2 from the
> intersection-point vertices using WKTReader().
> 1) >>> if Linestring1.intersects(Geometry):
> 2) >>>      pnts = Linestring1.Intersection(Geometry)
> 3) >>>      Linestr2_wkt = 'LINESTRING ( %s %s , %s %s ) % ( pnts.GetX(0)
>                          , pnts.GetY(0) , pnts.GetX(1) , pnts.GetY(1)  )
> 3) >>>      Linestring2 =
>                   geos.WKTReader(geos.GeometryFactory()).read(Linestr2_wkt)
> Question2: Is there a better way for building a new linestring by corpping
> old one with polygon, instead of this work around. ANd if not, how do I
> deal with GeometryPtr instead of Geometry, okay might be simple sub class
> or pointer issue this guy is not familiar with.
> thx in advance for any suggestions,
> Nico Hardebol


OGR geometries acquire the operations of GEOS geometries. I think you 
may not need to use the GEOS Python module at all. At any rate, the 
Python module in GEOS is not actively maintained.


Sean Gillies

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