Monks,

There are many threads on point-in-polygon analyses, and much published elsewhere. I know how to do it, but my problem is of speed. I am using the algorithm in the Wolf Book, and it works well. So, given an array of points (x,y pairs), and an array of polys (an array of array of points), I do the following --

POINT: foreach point {
  POLY: foreach poly {
    if point is in poly {
      do something
      last POLY
    }
  }
}
[download]

I have more than 5 million points, and more than 200k polys, leading to a possible 1 trillion intersections to be computed... obviously less than that because of last POLY, nevertheless, a pretty tedious task.

I seek advice on how to speed this up. Several thoughts come to mind, some difficult, some perhaps not so --

• Cache the polys... build a Berkeley DB while checking the first point, and from then on, test against the db instead of the polys file.
• Somehow compute where the point is likely to lie, so as to not do unnecessary comparisons... for example, what's the point of comparing cities in Maine with counties in California. To do so, I would have to build some kind of quadtree index.
• any other...

WWMD?

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