Wasn't it Stefan Viljoen who wrote:
>Hi all
>
>I have a problem using the trace macro - in the /advanced PovRay
>installation directory, there is a scene called isocacti that uses the
>built-in trace() macro to determine the "height" of ridges on a isosurface. 
>
>I am attempting to use the same technique in one of my scenes to place
>objects on the same isosurface (copied out of isocacti.pov), but trace()
>seems to detect the isosurface at least 3 units too "high" - i.e . all my
>objects "float" above the isosurface they are supposed to rest on. 
>
>Exchanging the isosurface in my scene with a plane object in the xy plan
>makes the trace macro in my scene work correctly - so the apparent problem
>is with trying to trace() an isosurface (although - how can it work
>correctly in isocacti.pov?)

The trace() function traces Ground at the position you #declared it.

You rotated and translated the Ground object after declaring it, but
trace() has no knowledge of that. 

Try applying the
         rotate <90,0,45>
         translate <50,-20,0>
commands inside the #declare Ground isosurface instead of the 
        object 
        {
                Ground
        }
section.

-- 
Mike Williams
Gentleman of Leisure

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