Another important point is that things like airfield and terrain elevation in aviation are measured in feet (in most places) and these feet are always the same absolute length. A “foot” on your altimeter is only on very rare occasions equal to a foot as you would measure it with a ruler, due to the fact that pressure and temperature have a role to play.
Great for separating aircraft on the same pressure setting from each other, but not so great at separating aircraft from terrain which is why there are safety margins built in, plus corrections for atmospheric conditions significantly below ISA.