setting data below ground to missing.

I have a grid of the form:

Time -> ((x,y,z) -> (param))

I also have a grid of topography (z) of the form:

(x,y) -> (z)

When I display the grid, I'd like to have all data
below the z surface be set to missing.

Is there a simple way to do this in VisAD without
going through each point manually?

Thanks.

Don
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Don Murray                               UCAR Unidata Program
dmurray@xxxxxxxxxxxxxxxx                        P.O. Box 3000
(303) 497-8628                              Boulder, CO 80307
http://www.unidata.ucar.edu/staff/donm
        "Time makes everyone interesting, even YOU!"
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