Shapely is an offshoot of the GIS-Python project that provides spatial geometry functions independent of any geo-enabled database. In particular, it makes python point-in-polygon calculations very easy.
Creating a Polygon
First, you need to create a polygon. If you already have an ordered list of coordinate points that define a closed ring, you can create a
Polygon directly, like so:
from shapely.geometry import Polygon poly = Polygon(((0, 0), (0, 1), (1, 1), (1, 0)))
But what if you just have a bunch of points in no particular order? Then you can create a MultiPoint geometry and get the convex hull polygon.
from shapely.geometry import MultiPoint # coords is a list of (x, y) tuples poly = MultiPoint(coords).convex_hull
Now that you have a
polygon, determining whether a
point is inside it is very easy. There’s 2 ways to do it.
point should be an instance of the Point class, and
poly is of course an instance of Polygon. within and contains are the converse of each other, so whichever method you use is entirely up to you.
In addition to point-in-polygon, you can also determine whether shapely geometries overlap each other.
poly.contains(poly) can be used to determine if one polygon is completely within another polygon. For partial overlaps, you can use the intersects method, or call intersection to get the overlapping area as a polygon.
There’s a lot more you can do with this very useful python geometry package, so take a look at the Shapely Manual as well as some usage examples.