to clean the interpolation of neighboring areas

[Isilda da Cunha Menezes]

Hello!
I would like to make a question to the users:

I have a shaded plot of station data, and the grads interpolated the
values to the vicinity of the country where I want to made the study and I
don't want that this happens. There's someone know how I can clean the
surrounding areas to that country for this not to appear in my shaded
plot?

I appreciate any hell

Isilda