Python – Using OGR Intersection Method Efficiently

gdalogrpython

I'm currently working to find the intersections between linears in a shapefile. I'm a newbie to the GDAL/OGR library. I've done some googling and came up with what seems to be a useful example(see below). There is a problem though. I don't see the method ExportToWkt(). My intellisense(or whatever it's called in IDLE) for the variable "intersection"doesn't have that option. Can anybody tell me what I'm doing wrong. Or just give me some general guidance to comparing two features in a shapefile and finding the intersection(if they intersect at all). Thanks.

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from osgeo import ogr

wkt1 = "POLYGON ((1208064.271243039 624154.6783778917, 1208064.271243039 601260.9785661874, 1231345.9998651114 601260.9785661874, 1231345.9998651114 624154.6783778917, 1208064.271243039 624154.6783778917))"
wkt2 = "POLYGON ((1199915.6662253144 633079.3410163528, 1199915.6662253144 614453.958118695, 1219317.1067437078 614453.958118695, 1219317.1067437078 633079.3410163528, 1199915.6662253144 633079.3410163528)))"

poly1 = ogr.CreateGeometryFromWkt(wkt1)
poly2 = ogr.CreateGeometryFromWkt(wkt2)

intersection = poly1.Intersection(poly2)

print intersection.ExportToWkt()

Best Answer

use the dir() function

dir(intersection)
['AddGeometry', 'AddGeometryDirectly', 'AddPoint', 'AddPoint_2D', 'Area', 'AssignSpatialReference', 'Boundary', 'Buffer', 'Centroid', 'Clone', 'CloseRings', 'Contains', 'ConvexHull', 'Crosses', 'Destroy', 'Difference', 'Disjoint', 'Distance', 'Empty', 'Equal', 'Equals', 'ExportToGML', 'ExportToJson', 'ExportToKML', 'ExportToWkb', 'ExportToWkt', 'FlattenTo2D', 'GetArea', 'GetBoundary', 'GetCoordinateDimension', 'GetDimension', 'GetEnvelope', 'GetEnvelope3D', 'GetGeometryCount', 'GetGeometryName', 'GetGeometryRef', 'GetGeometryType', 'GetPoint', 'GetPointCount', 'GetPoint_2D', 'GetPoints', 'GetSpatialReference', 'GetX', 'GetY', 'GetZ', 'Intersect', 'Intersection', 'Intersects', 'IsEmpty', 'IsRing', 'IsSimple', 'IsValid', 'Length', 'Overlaps', 'PointOnSurface', 'Segmentize', 'SetCoordinateDimension', 'SetPoint', 'SetPoint_2D', 'Simplify', 'SimplifyPreserveTopology', 'SymDifference', 'SymmetricDifference', 'Touches', 'Transform', 'TransformTo', 'Union', 'UnionCascaded', 'Within', 'WkbSize', '__class__', '__del__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattr__', '__getattribute__', '__hash__', '__init__', '__iter__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__setstate__', '__sizeof__', '__str__', '__subclasshook__', '__swig_destroy__', '__swig_getmethods__', '__swig_setmethods__', '__weakref__', 'next', 'this']

And you have all the class/function of the module. You may find what you are looking for with a little function adapted from a script Script de Python para filtrar por patrón de texto los métodos de Clases en PyQGIS de José Guerrero (already used in Iterating over selected features in QGIS Processing)

import re # regular expression
def get_patt(keyword, L):
    return [item for item in dir(L) if re.search(keyword,item)]

print get_patt(("Export",dir(intersection))
['ExportToGML', 'ExportToJson', 'ExportToKML', 'ExportToWkb', 'ExportToWkt']

print intersection.ExportToWkt()
POLYGON ((1208064.271243039052933 614453.958118694950826,1208064.271243039052933 624154.678377891657874,1219317.106743707787246 624154.678377891657874,1219317.106743707787246 614453.958118694950826,1208064.271243039052933 614453.958118694950826))
# or
print intersection.ExportToJson() # GeoJSON format
{ "type": "Polygon", "coordinates": [ [ [ 1208064.271243039052933, 614453.958118694950826 ], [ 1208064.271243039052933, 624154.678377891657874 ], [ 1219317.106743707787246, 624154.678377891657874 ], [ 1219317.106743707787246, 614453.958118694950826 ], [ 1208064.271243039052933, 614453.958118694950826 ] ] ] }

are correct

Rather than using ogr, use the Shapely module, it is easier.

from shapely.wkt import loads
from shapely.geometry import polygon, mapping
poly1 = loads(wkt1)
poly2 = loads(wkt2)
poly1.intersects(poly2)
True
print poly1.intersection(poly2).wkt
POLYGON ((1208064.271243039 614453.958118695, 1208064.271243039 624154.6783778917, 1219317.106743708 624154.6783778917, 1219317.106743708 614453.958118695, 1208064.271243039 614453.958118695))
# GeoJSON
print mapping(poly1.intersection(poly2))
{'type': 'Polygon', 'coordinates': (((1208064.2712430391, 614453.95811869495), (1208064.2712430391, 624154.67837789166), (1219317.1067437078, 624154.67837789166), (1219317.1067437078, 614453.95811869495), (1208064.2712430391, 614453.95811869495)),)}

And if you need to work with shapefiles, use the Fiona module (easier and also based on GDAL/OGR, with many examples in GIS stackexchange)

If you want to continue with osgeo/ogr, look at the Python GDAL/OGR Cookbook

Related Solutions

Shapely – Translating Between Polygons and MultiPolygons Using Shapely

Further to relet's answer on how to get individual polygons, you can then run an intersection on all the polygons to create the holes. If your dataset contains overlapping polygons though you're out of luck.

Explain again what is wrong with existing shapefile readers?

Would it not be easier to export feature IDs and M values from the shapefile and then join them back to the polygons after using an existing shapefile reader?

For multipatches you can use the same technique of assigning polygon IDs to a "patch ID" and then adding this attribute back to the features.

Edit: Whilst you say you don't want to use OGR, just in case you change your mind..

import ogr
# Get the driver
driver = ogr.GetDriverByName('ESRI Shapefile')
# Open a shapefile
shapefileName = "D:/temp/myshapefile.shp"
dataset = driver.Open(shapefileName, 0)

layer = dataset.GetLayer()
for index in xrange(layer.GetFeatureCount()):
    feature = layer.GetFeature(index)
    geometry = feature.GetGeometryRef()
    #geometry for polygon as WKT, inner rings, outer rings etc. 
    print geometry

The geometry should be output as follows:

POLYGON ((79285 57742,78741 54273...),(76087 55694,78511 55088,..))

The first bracket contains the coords of the exterior ring, subsequent brackets the coords of interior rings. If you have Z values points should be in the format 79285 57742 10 (where the last coord is a height).

Otherwise you could use the Shapely Contains and Within functions to assess every polygon with each other and apply a spatial index beforehand - http://pypi.python.org/pypi/Rtree/ to speed up processing.

[GIS] How to sort features in a layer using OGR/Python

I followed the second approach now and this seems to work, comments welcome:

def sortlayer(l, fd):
    # fids are unique, fids may be sorted or unsorted, fids may be consecutive or have gaps
    # don't care about semantics, don't touch fids and their order, reuse fids
    fids = []
    vals = []
    l.ResetReading()
    for f in l:
        fid = f.GetFID()
        fids.append(fid)
        vals.append((f.GetField(fd), fid))
    vals.sort()
    # index as dict: {newfid: oldfid, ...}
    ix = {fids[i]: vals[i][1] for i in xrange(len(fids))}

    # swap features around in groups/rings
    for fidstart in ix.keys():
        if fidstart not in ix: continue
        ftmp = l.GetFeature(fidstart)
        fiddst = fidstart
        while True:
            fidsrc = ix.pop(fiddst)
            if fidsrc == fidstart: break
            f = l.GetFeature(fidsrc)
            f.SetFID(fiddst)
            l.SetFeature(f)
            fiddst = fidsrc
        ftmp.SetFID(fiddst)
        l.SetFeature(ftmp)

Best Answer

Related Solutions

Shapely – Translating Between Polygons and MultiPolygons Using Shapely

[GIS] How to sort features in a layer using OGR/Python

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