I am planning a Drone Survey and the height of the drone will be 60m (196ft). So when I launch the drone it takes its datum height from the launch point as 0.0m (DJI M2P) it will then climb to said height and go off and do it’s thing.
However, one part of the survey is we are launching from the roof of a multi -story car park which is 12.31m (40ft) high. When I launch the drone it will have a datum height of 0.0m.
My question is, will the drone then ascend to 60m (above the building) and therefore 72.31m AGL or will it pick up its height from the GPS and go to 60m AGL.
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Your drone has absolutely no topographical data to know how high it is AGL … everything it measures is from take off, and all in-flight heights are measured barometrically from that take-off point.
GPS height is notoriously inaccurate compared to the horizontal plane.
Your drone will be logging the GPS height, but according to mine I have managed to fly under/through the ground … as has my car driven 50ft below the road surface and I’ve walked more than 100ft below a path … apparently. 
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Always fun on the beach when you’re flying over the sea at a reported height of -2m
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As always you have confirmed my suspicions and I have now reassessed my deployment plan accordingly. Thanks Team GADC
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