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[aprssig] filtering using shape files

Steve Dimse steve at dimse.com
Sat Mar 10 22:12:08 UTC 2007


On Mar 10, 2007, at 4:51 PM, Scott Miller wrote:

> I think you'd need to find a different solution for stream processing,
> though.
>
The algorithm is well known and it is easy to find code examples on  
the internet. Distilled, the idea is to draw an imaginary line  
segment between the point under test and an arbitrary distant point  
(known to be outside the shape, e.g. the north pole), then check  
mathematically to see if that imaginary line crosses each of the line  
segments in the polygon under test. If the number of crossings is  
odd, the point lies within the polygon. If the number of  
intersections is zero or even, the point lies outside the polygon.

Steve K4HG




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