Dres... I love dres, nice and brownish-grey (thanks to KASE), and has some craters, flat lands, canyons, its not too difficult to land on, has its own asteroids, it has all of the things.
true, a gps altimeter would probably be the best option (I totally forgot about those), but how would it give a reading to the ardruino? also, if the rocket is facing retrograde, which is the way it will be facing, thanks to the airbrakes, it will cancel out the horizontal velocity