[GRASS-user] Display 3-D volume

[Adam Dershowitz, Ph.D., P.E.]

I would like to calculate and draw a 3-D volume, but I am not really sure if nviz can do it, or how to accomplish it.  
In this case I have two surfaces that are both in space over a boundary.  In other words, there is a boundary on the "ground" and then there are two surfaces in space above them.  These surfaces are not parallel to each other.  The surfaces are defined using vector points.  
I am able to use v.to.rast to convert these surfaces to rasters and r.surf.nnbathy to fill in the two surfaces.  
I can then use mapcalc to find the local elevation difference between the two surfaces.  But that doesn't really solve my problem.  I want to be able to draw a 3-D image of this volume of space, where the top of the volume is the one of the initial surfaces, and the bottom is the other.  Then the horizontal extent should be the ground boundary.
If the above is not clear, then a simplified example would be to imagine drawing the airspace above someone's square property that goes from 1000 ft to 1500 ft.  The result would be a cube floating in space above the property.  In my case the shapes are more complicated, but the idea is the same.
Is there any way I can use grass to display this shape?  
It seems that nviz would need to have the shape as a vector shape, but I don't really see how I can calculate out this shape.
If anyone has any thought or ideas I would greatly appreciate hearing them.

Thanks,


--Adam