I'm new to all this but I have a question on how UAV determine their height above or below the launch site? I imagine on most flights, you place the UAV on the ground and arm it and it does a check on where it is and then you proceed to altitude above the launch site, but what happens if you start on a cliff and fly down to the beach? Does it not matter? Is the UAV able to calculate heights below its starting point?
Any simple explanation is welcomed.