On May 20, 2010, at 10:02 AM, Alexander Shenkin wrote:
Hello all,
I've been pouring through the various spatial packages, but haven't
come
across the right thing yet.
There is a SIG for such questions.
Given a set of points in 2-d space X, i'm trying to find the subset of
points in Y proximate to each point in X. Furthermore, the proximity
threshold of each point in X differs (X$threshold). I've constructed
this myself already, but it's horrificly slow with a dataset of 40k+
points in one set, and a 700 in the other.
A very inefficient example of what I'm looking for:
Not really a reproducible example. If euclidean_dist is a function ,
then it is not one in any of the packages I have installed.
for (pt in X$idx) {
proximity[i] = euclidian_dist(X[pt]$x, X[pt]$y, Y$x, Y$y) <
X$threshold
i = i+1
}
Have you considered first creating a subset of candidate points that
are within "threshold" of each reference point on both coordinates.
That might sidestep a lot of calculations on points that are easily
eliminated on a single comparison. Then you could calculate distances
within that surviving subset of points. On average that should give
you an over 50% "hit rate":
> (4/3)*pi*0.5^3
[1] 0.5235988
Perhaps crossdist() in spatstat is what I should use, and then code a
comparison with X$threshold after the cross-distances are computed.
However, I was wondering if there was another tool I should be
considering. Any and all thoughts are very welcome. Thanks in
advance.
Thanks,
Allie
--
Alexander Shenkin
PhD Candidate
School of Natural Resources and Environment
University of Florida
--
David Winsemius, MD
West Hartford, CT
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