Is it possible to set a RTH to be taken from GPS ?
I fly my Mavic in the mountains. When I start  my Mavic at the bottom of the hill and I will fly uphill it shows the height from the controller not from the terrain.
So eventually if I press RTH button when a drone is uphill (for example 100m height from where I stand) it will eventually descent to 50m, at might hit a ground.


Is there a way to change that setting to display actual height from the ground no from the controller ?

The return to home altitude is either your preset number OR your current altitude, whichever is higher.  If your aircraft is flying at 50 meters up the hill and your RTH is set to 30 meters, it will maintain 50 meters until directly overhead the RTH position.

To answer your question, I believe any height over about 5 meters above ground and the aircraft can't detect how high it is.  The bottom sensors don't work beyond that distance.