Originally Posted by
DaveReidUK
The software constrains the height AGL? How does that work in practice?
If the drone is using GNSS height, that implies it must have access to a terrain model to derive height AGL.
If it's using baro height, the same applies, with the added complication that the drone/controller needs to know the QNH.
Both sound a bit unlikely.
I should have been clearer.
The height is calculated above take-off point. (It's a bit more complicated than that, but you don't want a multi-page description).
The DJI drone has a barometer. Max flight time for the most popular model, the Phantom, is a bit over 20 minutes, so changes in local air pressure are not likely to be significant in that time.