No, I have not recently visited the local pot shop - there are several different ways to measure altitude.
Here is an excellent article from maping and GIS company Esri:
Mean Sea Level, GPS, and the Geoid
Frequently research and technology endeavors have unforeseen but positive outcomes. When European explorers set out to find a shortcut to India, they discovered the New World. When a staphylococci bacteria culture was mistakenly contaminated with a common mold, the clear area between the mold and the bacterial colony led to the conclusion that the mold, Penicillin notatum, produced a compound that inhibited the growth of bacteria. This chance discovery led to the development of the antibiotic penicillin.
That the earth does not have a geometrically perfect shape is well established, and the geoid is used to describe the unique and irregular shape of the earth. However, only recently have the more substantial irregularities in the surface created by the global mean sea level (MSL) been observed. These irregularities are an order of magnitude greater than experts had predicted. Controlled by the gravitational potential of the earth, these irregularities form very gentle but massive "hills" and "valleys." This astonishing finding was made possible through the use of GPS, a technology designed by the United States Department of Defense to revolutionize navigation for the U.S. Navy and Air Force. GPS has done that—and a lot more.
A lot more at the site - a well written article on a complex subject. Global mean sea level (what used to be the gold standard) is altered by the fact that the Earth's gravitational field is not homogeneous. A pocket of higher gravity will pull the sea level down to a lower level.
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