1-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.
For generations, the only way to express topographic or bathymetric elevation was to relate it to sea level. Geodesists once believed that the sea was in balance with the earth's gravity and formed a perfectly regular figure. MSL is usually described as a tidal datum that is the arithmetic mean of hourly water elevations observed over a specific 19-year cycle. This definition averages out tidal highs and lows caused by the changing effects of the gravitational forces from the moon and sun.
