help using a pre-loaded terrain file

Hi all

I have built some terrains using corel bryce - and I have loaded the terrain into my scene - my problem is i’m not sure how to implement floor collision detection

I know about setting up a bounding sphere and using collision rays for my avatar - but im not sure how to detect where my rays or sphere cross the vertices of my terrain - as the terrain is a loaded object and i don’t have a handle on it.

Anybody help??

Thank you

rmdire