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How far can the geoid rise above or fall below the best-fitting reference ellipsoid?

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Diagram showing the deflection of the vertical from the geoid to the ellipsoid or spheroid, with labeled lines for geoid, spheroid, rock surface, ocean, and geoid height in a geology context. — geoid Deflection of the vertical from the geoid to the spheroid.

The geoid is an irregular surface that represents the Earth's figure by coinciding with mean sea level over the oceans and continuing as an imaginary sea-level surface under continental areas. Because of gravity anomalies caused by irregular mass distributions and density variations within the planet, the geoid does not form a simple mathematical shape.123

To provide a practical reference for geodesy, scientists use a reference ellipsoid—a regular mathematical surface that approximates the geoid as closely as possible. The geoid fluctuates relative to this ellipsoid; it can rise up to 100 metres (330 feet) above the best-fitting reference ellipsoid and fall as much as 100 metres below it in other locations. These undulations are primarily attributed to mantle convection and temperature differences at depth, which correlate with plate tectonics and gravity anomalies.12