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Mean Sea Level

The gravity field of the earth is neither perfect nor uniform because density (and therefore mass) varies throughout the planet. This is due to magma distributions, mountain ranges, deep sea trenches, etc. If the Earth was a perfect sphere covered in water, the water would not be the same height everywhere. Instead, the water level would be higher or lower depending on the particular strength of gravity in that location.

The surface of the oceans that would exist if the oceans were in equilibrium - that is at rest, unaffected by wind, currents and moon and interconnected by canals across continental land masses - would be an equipotential surface.

MSL or Mean Sea Level is the average height of the ocean surface. Mean sea level is a significant datum reference for meteorological and altitude related pressure measurements, e.g. standard atmospheric pressure at mean sea level is 1013.25 hPa or 29.92 inHg.

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