thank you both for your fast response. Changing the projection to UTM did the trick. Now I can use the data in mason itself. It isn't important that that the projection is based on an ellipsoid. The area I'm using is round about 2x2km so euclidean plane is accurate enough.

Again thanks a lot. You're helping me a lot with my thesis!

a nice weekend wishes,

Am 14.06.2013 03:44, schrieb Mark Coletti:
[log in to unmask]" type="cite">On Thu, Jun 13, 2013 at 9:25 PM, Luís de Sousa <[log in to unmask]> wrote:
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:

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.