[gradsusr] Sub: mask file for US land/Ocean

[Adv]

Hi,
I would like to do the same calculation shown below, only over US land
region. Could someone help me with mask file ? or else explain me how to
use basemap shape file to mask the ocean. Is that possible to do?
An example of taking an area average of data only over land, given a mask
grid:

ave(aave(maskout(p,mask.3(t=1)),x=1,x=72,y=1,y=46),t=1,t=30)
say result

Thanks,
Adv
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