[wt200999]

Well with just the camera you can tell what point the target is at, and you can tell how big it appears. You know how big it should be and how high it should be. So compare the two. I am looking over this as well and it will probably be pathagerian therom like this:

(height)^2 * X^2 = (actual distance)^2

where height is the height in the picture compared to the actual trailer height and actual distance is a comparison between the size of the target and the apparent size of the target.

I think this is correct, but I am still thinking it over and working on it myself, I hope this helps you.