Dear Norman,

Calculating the area of an ellipsoid segment is something you probably don't want to do. Most GIS software does not implement it, those that do use approximations. You can get an idea of the complexity here:

http://www.math.auckland.ac.nz/Research/Reports/Series/539.pdf

You should do as Mark says, first project that geodetic data using an equal area projection and then feed it to GeoMason. This way you can use GeoMason as a GIS.

I've also found that, for whatever reason, a lot of data has attributes with those calculations already done for you. I don't know why this may be, but it doesn't hurt to check the attribute tables for your data to see if that is the case.