Sorry for the delay in responding Guys, I've spent a long weekend moving home and sadly no internet connection at the new place as yet.
I wasn't expecting such a detailed response, thank-you all very much.
As for CAB's input, what can I say except that I'm still trying to take it all in, not the easiest of tasks on a return to work - thank-you very much, but please let me sleep on some of those examples. (Beginning to think I've bitten-off more than I can chew
Serge is almost correct, it's not quite draping but interpolating levels from a TIN. I've found a great triangulation lisp routine on the web, "Triangulate" written by DANIELE PIAZZA and based upon some C code by the same name, credited to PAUL BOURKE.
I've written a small lisp app that creates 3 dimensional pipe & chamber networks using ACAD's solid modelling. My employer's GIS record system provides the Pipe Invert Levels and diameters, I just need to interpolate the Chamber Cover Levels to complete a totally automatic system.
With regard to the P4 issue, it was my intention to create a (Get_Z) function that takes a numeric X,Y argument, then scans through a selection set of 3D Faces before returning an interpolated Z elevation for that X,Y point.
Hope that makes sense.
Didge.