Hi Conrad,
The key thing to understand about Shapely's operations (intersection,
union, etc) is that they involve sets of points in space and not sets of
Python objects. A.intersection(B) returns a Python object that represents
all the points in the intersection of A's points and B's points.
If you want just a point-in-polygon test and to track your points
precisely, you could do something like this:
from shapely.prepared import prep
poly = prep(poly)
for xy in a:
if poly.intersects(Point(xy)):
# plot it
Yours,
On Wed, Jan 8, 2014 at 2:29 AM, Conrad Jackisch <coja...@gmail.com> wrote:
> dear community,
>
> i am struggling with a intersection of a large number of points with a
> polygon. i use shapely which helps already but returns only the coordinates
> of intersecting points of a MultiPoint object. is there anyone who could
> help with a way to simply get the indexes of intersecting points?
>
> - - - 8< snip - - -
>
> import numpy as np
> import matplotlib.pyplot as plt
> from shapely.geometry import Polygon
> from shapely.geometry import asMultiPoint
>
> poly=Polygon(zip([0.0,0.3,0.3,0.2,0.0],[0.2,0.2,0.4,0.5,0.4]))
> a = array([[0.1,0.1],[0.2,0.3],[0.15,0.45]])
> points = asMultiPoint(a)
>
> #plot example
> ax = plt.figure(figsize=(8, 4))
> ax = plt.subplot(121)
>
> patch = PolygonPatch(poly)
> ax.add_patch(patch)
> for p in points:
> ax.plot(p.x, p.y, 'o', color='#999999')
>
> pyplot.show()
>
> #intersection
> np.asarray(points.intersection(poly))
> #this gives me the coordinates of the values - but not the index, which i
> need much more.
>
> - - - snap >8 - - -
>
> of cause this example is very small and it was possible to compare the
> results with the given points. however, for large sets (about one million
> of points) this will slow down everything...
>
> if you have any idea, i was most happy.
> thanks,
> conrad
>
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>
--
Sean Gillies
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