Where max altitude is calculated from?

[gianni_BT]

The max altitude is calculated from the home point level or from the ground level below the aircraft?
And is it possible to fly below the home point level( e.g. if you start the flight from the top of a mountain)?

 

[Hummingbird.UAV]

Max altitude is calculated from your take-off point ie home point. Yes you fly down from there and get a negative altitude. Never heard of a limit on how far down you can go.

 

[old man mavic]

max altitude as shown on the app screen is calculated as above the home point once you fly away from the home point the reading on the screen has no relevance to your actual AGL
unless you were flying over a very flat landscape
in respect of flying below your home point ,into a canyon or deep valley then remember that as you go,lower and lower you run the risk of loss of enough sats for gps hold, and should the drone go into RTH due to loss of signal or some other reason,it will use a lot of battery just to climb to the RTH height set, before heading home so keep that in mind