How do flight computers in planes know how high above the ground they are.
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Main source is barometric altimeter. As was said it measures atmospheric pressure around aircraft, as you go higher pressure is lower and you can know your altitude.
Another source is Inertial Navigation System or Inertial Reference System. Very simply it is a “box” which knows how it is moved (it measures acceleration over time and know how fast it moves for how long and so it knows how it moved from its origin location). You tell the aircraft it is here. Then it knows how much it moves so it can determine its new location.
GPS. Not normaly used (or not used at all to get direct reading of altitude) but it is capadle to show you, theoreticaly.
Radioaltimeter. Something like radar, it emits waves to hit the ground and reflect back to the aircraft. Due to technical limitations it measures only up to 2000 ft above ground. This is the only instrument which gives you direct reading of real height about ground.
Aircraft do not use height above ground but “height above sea level” which is called altitude. If you land, your instruments do not show 0 but show the elevation of airport above sea level.
Barometric altimeter is fundamental instrument for all aircraft. GPS can be found in small aircraft if they are equiped for flying in Instrument meteorological conditions (you cant see because of bad weather). Radioaltimeters and INS/IRS are just for big aircraft, those are very expensive things.
Height above terrain can be obtained via combination of all known data and map database.
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