I'm looking for a way to match people so as to find other members that are 'nearby' - not for those dreadful 'girls near you' ads, don't worry ;_)
Of course 'nearby' in the country is a different thing than when in the city.
I'm thinking that an efficient way to do this would be to use a quadtree for their location, but use deeper level divisions when the population density is higher. In other words try to 'balance' the tree so that every leaf node has roughly the same population within it.
I was thinking that someone may have already done this but can't find such a thing on the web.
If not, is there an alternative approach I should look into?