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• Sea level seems like a pretty easy concept, right? You just measure the average level

• of the oceans and that's that. But what about parts of the earth where there aren't oceans?

• For example, when we say that Mt. Everest is 8850m above sea level, how do we know what

• sea level would be beneath Mt. Everest, since there's no sea for hundreds of kilometers?

• If the earth were flat then things would be easy - we'd just draw a straight line through

• the average height of the oceans and be done with it. But the earth isn't flat.

• If the earth were spherical, it would be easy, too, because we could just measure the average

• distance from the center of the earth to the surface of the ocean. But the earth isn't

• spherical - it's spinning, so bits closer to the equator are "thrown out" by centrifugal

• effects, and the poles get squashed in a bit. In fact, the earth is so non-spherical that

• it's 42km farther across at the equator than from pole to pole. That means if you thought

• earth were a sphere and defined sea level by standing on the sea ice at the north pole,

• then the surface of the ocean at the equator would be 21km above sea level.

• This bulging is also why the Chimborazo volcano in Ecuador, and not Mount Everest, is the

• peak that's actually farthest from the center of the earth.

• So how do we know what sea level is? Well, water is held on earth by gravity, so we could

• model the earth as a flattened & stretched spinning sphere and then calculate what height

• the oceans would settle to when pulled by gravity onto the surface of that ellipsoid.

• Except the interior of the earth doesn't have the same density everywhere, which means gravity

• is slightly stronger or weaker at different points around the globe, and the oceans tend

• "puddle" more nearer to the dense spots. These aren't small changes, either - the level of

• the sea can vary by up to 100m from a uniform ellipsoid depending on the density of the

• earth beneath it. And on top of that, literally, there are those pesky things called continents

• moving around on the earth's surface. These dense lumps of rock bump out from the ellipsoid

• and their mass gravitationally attracts oceans, while valleys in the ocean floor have less

• mass and the oceans flow away, shallower.

• And this is the real conundrum, because the very presence of a mountain (& continent on

• which it sits) changes the level of the sea: the gravitational attraction of land pulls

• more water nearby, raising the sea around it. So, to determine the height of a mountain

• above sea level, should we use the height the sea would be if the mountain weren't there

• at all? Or the height the sea would be if the mountain weren't there but its gravity

• were?

• The people who worry about such things, called geodetic scientists or geodesists, decided

• that we should indeed define sea level using the strength of gravity, so they went about

• creating an incredibly detailed model of the earth's gravitational field, called, creatively,

• the Earth Gravitational Model. It's incorporated into modern GPS receivers so they won't tell

• you you're 100m below sea level when you're in fact sitting on the beach in Sri Lanka

• which has weak gravity, and has allowed geodesists themselves to correctly predict the average

• level of the ocean to within a meter everywhere on earth. Which is why we also use it to define

• what sea level would be underneath mountains... if they weren't there... but their gravity

• was.

Sea level seems like a pretty easy concept, right? You just measure the average level

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B1 sea level sea earth level gravity height

# What is Sea Level?

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Bing-Je posted on 2013/12/08
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