Kinect? (
http://www.youtube.com/watch?v=4EGQZ...eature=related)
Failing that, maybe a poor man's kinect? Maybe with a strong IR emitter on the bottom and an IR sensor, you could determine from the brightness of the received IR how far away from the ground you are. This would depend on how much the ground below absorbs IR so it might not work if you don't know what you'll be flying above.