Thanks, now I see what is happening.
Don’t know why Paul add that condition of if -I than no -Q, which doesn’t seem to be necessary. If I comment it in the code, I get a PS like this
but converting to png or jpg those blacks disappear and we see what’s under them.
But you can save your day using grdview -Qc. This (and I odified the z.cpt to have a B,F,N all equal to white)
gmt grdview Chain_Bathy_100m_neg.grd -R16.3W/12.8W/2.2S/0.3S -I+d -Cz.cpt -JM15c -Qc -P -K > lixo.ps
gmt grdview MGL2411_multibeam.grd -R -I+d -Cz.cpt -JM -Qc -O >> lixo.ps
gives similar to your Chain_25b_MGL2411_ProgressUpdated.ps