So what about the Barometric sensor on board? It measures altitude based on air pressure similar to commercial aircraft. It’s not 100% accurate and in the commercial world the pressure at the runway altitude is taken as a reference. Not used for landing though - there are cases where aircraft have crashed when the altitude of the runway has not been correctly set.
Then there is the altitude calculated by the GPS.
Barometer just measures pressure. Crudely this is just altitude. When calibrated height above sea level (QNH) or far less commonly, aerodrome level (QFE). Or standard pressure settings in higher class controlled airspace.
Altitudes ATC and flight planning wise are always given above calibrated mean sea level unless states (and its very rare to do anything else in normal operations).
It tells you nothing about the immediate height under the aircraft at all - it cant. But it doesn't have to. Its not important.
In pretty much all manned aviation QNH/Barometric is used on the approach, calibrated to mean sea level. Including for pretty much every standard and instrument approach procedure. Its also used in the flight levels and everything else. Baro altitude IS the gold standard. So yes it is used for landing, almost all the time.
Circuit heights, glideslope checks,decision heights and everything else are generally ALL given as barometric above sea level so it is "used for landing". GPS is pretty much never used (and GPS isnt a mandatory requirement even for aircraft, a barometric altimeter is).
Because actual height over the ground directly beneath you is something you very very rarely need to know.
You don't land a plane based solely on the altitude readout. You land it by looking out of the window and/or following instruments and charts. You also have a declared minimum altitude where if you cant see the runway visually you go around.
Yes mis-calibrations are possible but that-s why every part of a flight has checklists and crosschecks to significantly reduce the error.
The Mavic calibrates its barometer at the takeoff spot. So it basically sets QFE. So it tells you the barometric height above takeoff point which is not the same as ground. The VPS alone can tell you height above ground and they only work at 20-30ft or so.
As for GPS altitude, forget it. Its inaccurate and thats before you dial in the various datum, geoid corrections and so on. Simple arent going to get a GPS anywhere near accurate in altitude in a cheap consumer drone to make it worth switching from the barometer....and obviously it needs a good GPS signal to even work at all.