The way we measure altitude low to the ground normally depends on how far that ground is from Mean Sea Level (MSL) since we are usually dependent on barometric altimeters to figure that out. But what is Mean Sea Level?
The odd thing is that the level of the sea is different all over the world because of local variation of the earth's magnetic field, but many occupations, including ours, need to have a frame of reference. So even though it is rarely correct in terms of, yes that is where the sea's level is, we have a standard. If you turn your altimetry to 29.92 inches of mercury on an ISA standard day, where it reads 0 is where mean sea level is. So we are defining mean sea level based on the definition of mean sea level. Nuts? It really doesn't matter so long as you understand it is a theory of convenience.
Everything here is from the references shown below, with a few comments in an alternate color.
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