Thanks for the reply.
It is a neat idea, but in my case i am using co-ordinates x and y.
I could use a polygon region system which will count number of positions in an area, but if the map image underlay changes, then i need to recreate the polygons.
Hmm... I don't deal with maps enough to know for sure, but it seems to me there must be a way. Hopefully someone else can jump in and answer.