I have to assume from what you say that you don't have a DB matrix terrain as you said it was done in Geoscape 3D. What you have is a 3D model terrain.
You also didn't say what version of DB you have (like it clearly states in the stickies at the top of the newcomers board). This is because the answer to questions may be different for the two versions of DB you see.
Anyway, I'm not assuming anything about your programming experience, but if you are a newcomer then you have just discovered why DB's matrices are much easier to use when you are learning - although, they aren't quite as pretty!
You will have to learn about collision and implement it yourself with the terrain you are using. DB only provides commands for it's own matrices.
You treat your terrain like any other 3D object and use the collision commands.
TDK_Man