I did analysis of a NetCDF file that cover the whole of West Africa but I need the fill the data over Niger river Basin using shapefile (polygon). I could only see a x-point example on ferret documentation page.
I think there have been similar discussions in the Ferret mailing list:
Please search the archive with "shape file mask" (without the double quote symbols). I myself am not familiar with the subject at all.
Regards,
Ryo