I keep try to explain this to people and have never been able to make it this clear. Geodesy 101.
The Surprisingly Complex Way That Scientists Measure Sea Level

Most of us have probably never given a second thought as to how sea level is determined. It’s basically where the ocean starts, right? Not even close. It turns out there are countless factors that have to be taken into account because the oceans simply aren’t at the same level all around the world.

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