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Benchmarks and Sea Level

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Although on this site we use the term benchmark generically, to indicate all survey stations (disks, nails, water towers, etc.), technically, as I understand it, a benchmark indicates a point where elevation is determined.


Originally, I believe, this represented distance above sea level. Which gets to the question of what, exactly, constitutes sea level, and how do you measure it.


It turns out that sea level changes over time, and it's different depending on where you measure it. For example, the Atlantic is about 40 cm lower than the Pacific, but currents and gravity create differences within each ocean.


An entity called the Permanent Service for Mean Seal Level based in Britain coordinates all the measuring and calculating, and they have some very interesting information on their web site.


A good place to start is on their FAQ page.


But now, it appears that modern datums (those pesky things like NAD83 and WGS84 that represent mathematical models of the shape of the earth) are three-dimensional and provide elevation figures that significantly differ from measurements with respect to water level. I found a helpful document on the web site of the Office of Coast Survey.


OK gang, now study up; a pop quiz could happen any time :-)

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Elevations are usually stated with respect to a GEOID model (model of gravity, and hence mean sea level), not the ellipsoid. Even handheld GPSs normally report altitude with respect to a GEOID.

(if you've played with the raw outputs with some of them, you can sometimes get the height-above-ellipsoid or the geoid correction)

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