collision detection

hi all,

i have one terrain (consists of hills and low land). when i press the up arrow key, the ‘camera’ will move onwards.

my problem is HOW TO CODE:
when the camera meets certain coordinates, for eg hill, then the camera will go up. when it meets coordinates for low land, then the camera automatically will go down?

i really hope that anybody can gimme sorce codes, so that i can look through it, and make some references.

thanks in advance.