All altitudes maintained by the bird are relative to the take off or Home point. So readouts and settings like Return To Home (RTH) are all reference from that point. For example, if your takeoff point is at 900 ft Above Sea Level (MSL), the bird sets that internally as '0' ft. once you climb to 100 feet above the home point (above Ground Level or AGL), that's what the bird will report. In global terms, you would actually be at 1000 (900+100) ft MSL. Now as you fly out but don't climb, if the ground rises up by 50 ft, you are now at 50 ft AGL, still 1000 ft MSL and the bird still reports 100 ft. The altitude is monitored using an internal barometer just like manned aircraft and doesn't actually measure the height AGL - that would require a range finding altimeter such as a radar or laser radar. All the above goes to illustrate why you have to be careful when setting your RTH heights and be very aware of obstacles like trees and buildings as well as changes in the terrain elevation. a 50 ft tree on ground at the same level as your home point can easily be passed over with a 75 ft RTH, but if that same tree is on a hill that is 40 ft higher that you home, the top of that tree is now 90 ft above the home and your bird could try to fly right through it on a RTH (note that I said 'try' these birds are notoriously bad at flying though solid objects!). Hope this helps.