This tip provided by Peter Ashwood-Smith contains a Windows executable and C++ source code for computing the Latitude and Longitude of contest box markers.
The file, GPSBox.exe is a Microsoft Windows(R) executable. It will open a command session like the one in the picture. You enter the latitude and longitude of the center-front marker followed by the true heading (not magnetic) facing right along the front of the box, along the X axis, upwind on a B flimsy.
The program provides a list of waypoints in a format suitable for upload to your GPS unit, including a location for the judges. It also outputs a map showing the location of the waypoints.
This file, GPSBox.cpp contains the source code for the program. The file, make.bat contains commands to compile the source code.
IAC Chapter 38 has a browser application from Peter Jensen. The application works with IE 6.0 but does not work with FireFox 1.5. The numbers given by Peter Ashwood-Smith's program match the numbers given by Peter Jensen's to 0.004 minutes. One program is using meters and the other feet.
According to David L. Wilson, the GPS gets you within a few meters, depending on the unit. (Google GPS accuracy.)
This page on the FCC web site computes distance between two points specified using lat and lon, degrees, minutes, and seconds. The distance is the great circle distance.
Peter Jensen wrote to me with a source for the formulas used in the on-line calculator. The referenced page, Aviation Formulary by Ed Williams , contains a wealth of physics and engineering formulas applied to aviation.