Geoid
In space and astronautical engineering, geoid refers to the equipotential surface of the Earth's gravity field that best fits the global mean sea level. It is an imaginary surface that extends continuously under the continents. The geoid is used as a reference surface for measuring elevations and gravity anomalies. It is an important tool for satellite altimetry, which measures the height of the ocean surface relative to the geoid. The difference between the geoid and the ellipsoid, which is an idealized mathematical model of the Earth's shape, is called the geoid undulation. The geoid undulation is positive in areas where the gravity field is stronger than the global average and negative where it is weaker. The geoid undulation is an important parameter for determining the accuracy of satellite navigation systems and for studying the Earth's interior structure.
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