How to trim an area in a general map?

[BrunoB]

Hello all,

I have a GENERAL MATRIX MAP, in latitudes and longitudes coordinates.
I would like to trim this map according to the latitude and
longitude vectors of a basin inside that zone.

I can't seem to do it with 'roipoly' because its not a REGULAR matrix
map, nor with 'maptrim' because its not a vector map.

Any clues?

Thanks a lot,

Bruno