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This one really got us thinking. And around here thinking quickly leads to weeping, followed shortly thereafter by disconsolate Twinkie-eating. Luckily, in this case, the Web bailed us out, providing several resources for information on the elusive concept of "sea level." First, we did a search on the phrase "calculating elevation," which seemed like it might narrow things down more than "sea level." One of the first results landed us smack dab in the middle of the National Center for Geographic Information and Analysis's Core Curriculum for Technical Programs, where John Schaeffer describes using particular "datum planes" when determining elevation via the Global Positioning System (GPS). It
seems that a great deal depends on whether you're using a geoid model or an ellipsoid model. Confused? Don't worry. So were we. Here's the short course: Sometimes elevation above sea level is determined by the geographic feature's height above "mean sea level" -- basically the average height of all of the oceans. While this may be pretty good for determining the relative height of all geographic features worldwide, it's really not all that accurate, since yes, as you mentioned, sea level isn't the same all over. For a more accurate determination of local elevations, scientists are forced to create mathematical models of the sea's curvature. That's what Schaeffer is talking about in his lecture on datum planes. Of course, that got us wondering if anyone has thought to map the
ocean's topography -- the sea's elevation above (or below) sea level, if you will. Sure enough, after a few quality clicks we spied TOPEX/Poseidon, a joint U.S.-French effort to "measure global sea level." By exploring the site, we learned that the TOPEX satellite can determine the height of the ocean to an accuracy of five centimeters. Not bad for a tin box circling 830 miles overhead.
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