That is the theory but if you do fly over a cliff it's only reference is GPS location and air pressure. If you have noticed the pressure may alter so if the pressure is higher than take off no problem because it will clime to 120ft above take off pressure and fly to GPS location and land. Now if the pressure lowers in the valley it will try to plant it's self in the side of the cliff. Bearing in mind air pressure is affected by many factors, wind only being the effect of pressure differences not the cause.So In theory, as long as you take off at a level of say zero difference or its reference start point, if you went -60 feet, it "SHOULD" climb to 120 feet to come and land back at its start point of 60 feet.