Greetings Eric,

The simplest solution for a few reference points is to use Google Earth to determine the coordinates for a particular spot on earth. If you are looking at mapping a large number of points and depending on your application, a variety of GIS software applications are available and would be the route to take.

As for the math, interpreting the coordinate is a matter spacing between perpendicular reference points, as close to perpendicular as possible anyway, with curving references. Its the same as it would be with a paper map, which while flat still has curving reference points, hence the plethora of compass rose references on a sectional chart. There is also the matter of scale/resolution.

With a little more about the application you have in mind I could provide more useful information.

~Jon