I am a geologist, I imported geological surfaces called faults into Rhino3D to be able to convert them into solids so I can use them in some CAD software packages.
The problem is that I have the depth of the faults in positive values, so Rhino3D displays them above the ground surface
1- Is there anyway of switching the Z values to minus within Rhino3D?
2- What is the easiest way to convert them into solids as they are meshes in the DXF format.
1) You should be able to mirror them about the XY plane, that effectively inverses the Z values. From the Front view, select your geometry, type Mirror (Copy=No), type 0 for the origin and with Ortho on, pull the cursor out along the X axis somewhere and click. That will give you a Z axis mirror. The following macro automates the process (you can put it on a button):
_Cplane _World _Front _Enter
_Mirror _Pause _Copy=no
If you are importing the data for creating your meshes in some kind of point format, it may be possible to invert the Z coordinates on import with a script.
2) I'm afraid there is no easy way to convert meshes to solids, especially something as complex and chaotic as terrain data. You will probably need to do some manual remodeling. There might be some useful info here.
If the meshes are low-poly, you might be able to get away with using MeshToNurb to convert each mesh facet into a NURBS surface.
I will try these and let you know