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

N.J. Hardebol nico.hardebol at falw.vu.nl
Wed Feb 28 16:51:46 EST 2007


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

In first problem, I start with WKT or WKB geometry strings retrieved from
postgis database. Afterwards I want to parse them to geos Geometry
>>> 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 =

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

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