# Point-in-polygon using Python and OGR

I want to use OGR in Python to write a simple point-in-polygon test:

``````def point_in_polygon(point, polygon):
"""
point : [longitude, latitude]

polygon : [(lon1, lat1), (lon2, lat2), ..., (lonn, latn)]

"""
# Create spatialReference
spatialReference = osgeo.osr.SpatialReference()
spatialReference.SetWellKnownGeogCS("WGS84")
# Create ring
ring = osgeo.ogr.Geometry(osgeo.ogr.wkbLinearRing)
# Add points
for lon, lat in polygon:
ring.AddPoint(lon, lat)
# Create polygon
poly = osgeo.ogr.Geometry(osgeo.ogr.wkbPolygon)
poly.AssignSpatialReference(spatialReference)
poly.AddGeometry(ring)
# Create point
pt = osgeo.ogr.Geometry(osgeo.ogr.wkbPoint)
pt.AssignSpatialReference(spatialReference)
pt.SetPoint(0, point[0], point[1])
# test
return pt.Within(poly)
``````

However, it doesn't seem to work properly:

``````In [22]: polygon = [(-10., -10.), (-10., 10.), (10., 10.), (10., -10.), (-10., -10.)]

In [23]: point_in_polygon([0., 0.], polygon)
Out[23]: True

In [24]: point_in_polygon([359., 0.], polygon)
Out[24]: False
``````

Any ideas what I'm missing?

Thanks for your insight :)

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## 2 Answers

I suspect it's because the underlying GEOS library only work in Cartesian space rather than spherical, so you'll have to subtract 360 from any longitudinal coordinate greater than 180, which makes 359 == -1. Of course, you'll still have problems with features crossing the anti-meridian (i.e. +- 180 degrees longitude), but you can easily detect that and not do the 360 subtraction step.

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Thanks for your clarification. It strikes me as odd that something having "geospatial" in its name doesn't cover these simple geo-applications. Any other idea how I can solve the point-in-polygon in Python without having to consider those cases manually? – andreash Apr 17 '12 at 6:40

According to your comment if you are looking for pure code rather using OGR package, there are many good links such as this: in C and this: in Python which directly address your problem by pure codes. You may find even more by googling;)

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thanks, but those links don't take into account that the Earth is a sphere. – andreash Apr 30 '12 at 11:48