Thanks for the info, I'll check it out.
Urb
You almost certainly want to use the Haversine formula for this task
(calculating distances to points that are quite close on a sphere).
http://en.wikipedia.org/wiki/Great-circle_distance
The basic Great Circle formula gives *incorrect* results for points that are
very close to each other. I.E., the zip code 05702 is NOT actually on top
of 05701, and I expect that your database of zip code centroids is fine,
however, at some point roundoff error is killing you. That is,
arccos(1) = 0 //true!
arccos(.999999999) = 0 //true for computers, not true for mathematicians
So zip 05702 is too close to zip 05701, and its arccos is getting rounded to
zero, which makes the distance calculated be zero, which is untrue.
Use one of the other formulas on that Wikipedia page.
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