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How to calculate the area of a polygon on the earth's surface using python?

Asked 2011-01-13T15:26:29.843
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The title basically says it all. I need to calculate the area inside a polygon on the Earth's surface using Python. Calculating area enclosed by arbitrary polygon on Earth's surface says something about it, but remains vague on the technical details:

If you want to do this with a more "GIS" flavor, then you need to select an unit-of-measure for your area and find an appropriate projection that preserves area (not all do). Since you are talking about calculating an arbitrary polygon, I would use something like a Lambert Azimuthal Equal Area projection. Set the origin/center of the projection to be the center of your polygon, project the polygon to the new coordinate system, then calculate the area using standard planar techniques.

So, how do I do this in Python?

python geometry geolocation geospatial
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Because the earth is a closed surface a closed polygon drawn on its surface creates TWO polygonal areas. You also need to define which one is inside and which is outside!

Most times people will be dealing with small polygons, and so it's 'obvious' but once you have things the size of oceans or continents, you better make sure you get this the right way round.

Also, remember that lines can go from (-179,0) to (+179,0) in two different ways. One is very much longer than the other. Again, mostly you'll make the assumption that this is a line that goes from (-179,0) to (-180,0) which is (+180,0) and then to (+179,0), but one day... it won't.

Treating lat-long like a simple (x,y) coordinate system, or even neglecting the fact that any coordinate projection is going to have distortions and breaks, can make you fail big-time on spheres.

answered 2011-01-13T17:10:26.883

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