Masking out a variable below a particular value using Grads

How to mask out a certain region below a particular water depth or above a particular elevation with dataset. is a global topographic and bathymetric chart with 15 arc-sec resolution which can be obtained from

Below is an example to mask out the ocean part with water depth below 200 m.

q file
set lon 128 147
set lat 30 46
set mpdset hires
set map 1 1 10
set gxout shaded
d elevation
define d200 = maskout(maskout(elevation, elevation-(-200)),0-elevation)
d d200
open surf_xxxx.ctl
q file 2
define mask = const(d200,10,-u)
d mask
set t 1
set z 1
define maskdata=lterp(mask,u.2(t=1))
d maskdata
d maskout(u.2(t=1),-maskdata)  
draw title Wind resource within 200 m depth