grdclip - Clipping of range in grdfiles. grdclip input _ file . grd -G output _ file . grd [ -Sa high / above ] [ -Sb low / below ] [ -V ] grdclip will set values < low to below and/or values > high to above . ... Sa Set all data[i] > high to above . Sb Set all data[i] < low to below . ... To set all values > 70 to NaN and all values < 0 to 0 in file data.grd, try: grdclip data.grd -G new_data.grd -Sa 70/NaN -Sb 0/0 -V gmt , grdlandmask , grdmask , grdmath , grd2xyz , xyz2grd . ...