What is Sea Level?

Sea

level

seems simple at first, right? It simply measures average ocean

level

s. But

what

about the parts of the Earth where there are no oceans? For example, when we say that the top of Mount Everest is 8,850 meters above sea level, how do we know

what

the sea level will be below Mount Everest, since there are no seas for hundreds of kilometers? If the Earth were flat, things would be easier: we would simply draw a straight line through the average height of the sea and the problem would be over. But the Earth is not flat. If the Earth were spherical, the process would also be simple, because we could simply measure the average distance from the center of the Earth to the surface of the ocean.

But the Earth is not spherical. The Earth is rotating, so the parts close to the equator will be "ejected" due to centrifugal force and the poles will be slightly compressed. In fact, the Earth is far from spherical, being 42 km farther away at the equator than the distance between the poles. This means that if you believe the Earth is spherical and you define the sea surface by being on top of sea ice at the North Pole, then the ocean surface at the equator will be 21 km higher than sea level. This bulge is also the reason why the Chimborazo volcano in Ecuador, rather than Mount Everest, has the peak farthest from the center of the Earth.

So how do we determine sea level? In fact, water is bound to the Earth by gravity and therefore we can model the Earth as a flat, elongated sphere, and then calculate the height at which the ocean will be stable when pulled by gravity. . on the surface of this elliptical shape. Except that the Earth's interior doesn't have the same density everywhere, which means that gravity is slightly stronger or weaker at different points around the Earth, and that the ocean tends to "concave" more toward points. of density. These are not minor changes either: sea level can vary by up to 100 meters in a regular ellipse depending on the density of the land below.

On top of that, there are literally these annoying things called continents that move around the surface of the Earth. These dense masses of rock protrude from the elliptical shape of the Earth and their masses attract the oceans with their gravity, while the grooves of the ocean floor have less mass and the oceans move away from them, becoming shallower. And here lies the real mystery, because due to the strong presence of a mountain (and the continent on which it is located), the sea level changes: the gravitational force of the earth attracts more water towards it, raising the sea level around it. .

So to determine the height of a mountain above sea level, should we use the height of the sea if the mountain had not existed in the first place? Or would the sea rise without the presence of the mountain, but with the continuous influence of its gravity? Some people concerned about such a situation, called surveyors, decided that we should actually determine sea level by taking into account the force of gravity, and they even came up with a surprisingly accurate model of the Earth's gravitational fields, which they cleverly called: the Earth's gravity model. It has been integrated with modern GPS receivers so that it doesn't tell you that you are 100 meters below sea level, even though you are actually sitting on a beach in Sri Lanka where gravity is weak.

In addition, it allowed the surveyors themselves to correctly estimate the average rise in ocean level per meter across the entire Earth. That's why we also use the model to determine what the sea level beneath the mountains should be... if they weren't there in the first place... but their gravitational pull still exists.