computing a parameter at a single location

I need to be able to compute the gradient between two points for which I
will only know the lat/lon values.  I'm looking for the easiest way to do
this.  Is there a way to spit out the value for a parameter by providing a
single lat/lon? Or alternatively, is there a simple way of obtaining the
actual gridpoints for a location if I only know the latitude/longitude
coordinates so that I can easily reference the grid points in a Fortran

Matt Borkowski
Graduate Student
North Carolina State University
Department of Marine, Earth, and Atmospheric Science