[nl]

You could/should make the URL be a GeoHash [1]

GeoHash's have the extremely attractive property that shorter versions of the same URL correspond to larger areas around the same point - ie, you can trade precision for size.

[1] http://en.wikipedia.org/wiki/Geohash#Example

[dave1010uk]

Geohash also doesn't require a database to lookup hashes - all you need is the formula. The only disadvantage seems to be the slightly longer URLs.