I modified your script a bit and I get this map. I save the data in a file pamp.dat.
Full script:
gmt begin South_Africa pdf
gmt set MAP_FRAME_TYPE plain
gmt set FONT_LABEL 16p
gmt makecpt -Cjet -T-15/30/2
gmt coast -R16/33/-35/-22 -JM20c -BWSne -Baf -I1/1p,0/200/251 -W1,black -Gwhite -Di -N2/0.25p,black -Na/0.5,black -Swhite
gmt surface pamp.dat -Gsurface.GRD -T0.5 -I0.01 -R16/33/-35/-22
gmt grdimage surface.GRD -Q -C
gmt coast -Ir/grey -Wlightgrey -Swhite -N1/darkgrey
gmt plot pamp.dat -Sc0.1
gmt end