http://maplepark.com/~drf5n/cgi-bin/imagemapInterpolate.cgi?ULdxdy=-78.6366,40.0835,.0081107,-.008052&coords=

This program, imagemapInterpolate.cgi, was written by David Forrest, way too late at night You may want to look at how it uses cgi-lib.pl and other things.

-78.6366,40.0835,.0081107,-.008052
-78.6366 + * .0081107, 40.0835 + *-.008052 = --> -78.6366,40.0835