Definition
The geoid is the equipotential surface of the Earth's gravity field that best matches undisturbed mean sea level, and it undulates by up to roughly a hundred metres relative to the reference ellipsoid because of density variations. The separation between geoid and ellipsoid, called geoid height or N, is why GNSS altitude differs from height above sea level, and why vertical datum transformations require a geoid model as well as a horizontal datum.
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