[aprssig] Proximity detection

[Robert Bruninga]

> Or you could use cosine corrected Pythagorean theorem.... 

APRSdos uses that, and that is why its maps are not elongated at
higher latitudes as are some other clients.  It's trivial to
multiply all longitudes by a simple constant to make the maps
more realistic.

Here is the simplified distance calculation you can use in a PIC
if all you are trying to do is estimate relative distance.  Its
good to about 80% relative accuracy:

http://www.ew.usna.edu/~bruninga/aprs/APRS-Distance.GIF

Bob, WB4APR