Hi jbgibson, thanks for your welcome and your answer to my question.

Yes, your explanation definitely makes sense. My base map is an equirectangular vector map made in Inkscape. Even though Inkscape lets me measure areas, the map projection is unfit for that purpose. Somehow I didn't make the leap to transform it to an equal-area projection. I have the Gimp on my computer, but I don't use it that much. I'll have a look at it to see how I can get some results.

Thanks again for the hint!

Cheers, Akubra