
Differences Between MSL and GPS Altitudes - gbacon
https://www.esri.com/news/arcuser/0703/geoid1of3.html
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gbacon
From the article:

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.

