I think it's out of MoI's reach to do this with any degree of accuracy. Ground has arbitrary topology and so it's best to use a mesh. Would need some tool like Rhino's Delauney triangulation described here:

http://en.wiki.mcneel.com/default.aspx/McNeel/PointsetReconstruction.html

Or in Moi - I suppose you could try making a series of cross-sections through those topo curves, and loft them to a surface. OR loft portions of the original curves until you've got individual surfaces making the hills and valleys, then trim them and join to a polysurface. But I think either method would be a very inaccurate... Another option would be a patch surface with a LOT of control points, or possibly use a mesh patch. But neither option are available in MoI...

jonah