How do flight computers in planes know how high above the ground they are.

Question

How do flight computers in planes know how high above the ground they are.

In:

Answer 1

There are several methods
1: Most planes have radar made specifically for that purpose. However, this only shows you how far off the ground you are, so if youre flying at 5100 meters, starting from 5000, radar will show you 100 Meters.
2: Though this isnt uniform as air pressure can vary considerably on a local basis, the pressure around you gives a pretty accurate idea of how high you are. Though, this has the opposite drawback of radar. Whereas radar will tell you how far above the ground you are, this method shows your height relative to sea level.