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How is the geoid related to Earth’s gravitational field?

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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 fundamentally defined by Earth’s gravitational field. It is an equipotential surface, meaning that the potential function—which accounts for both the gravitational attraction of Earth’s mass and the centrifugal force caused by its rotation—remains constant across its entire extent.1

Because the geoid is everywhere perpendicular to the pull of gravity, the local direction of gravity is always normal to its surface. While the geoid coincides with mean sea level over the oceans, it is not a simple mathematical shape. Its surface is irregular due to gravity anomalies caused by the uneven distribution of mass and density heterogeneities within the planet.12

In geodesy, the geoid serves as a critical reference surface for measuring precise elevations. Because spirit levels are set tangential to equipotential surfaces, they effectively measure height relative to the geoid rather than a simple geometric ellipsoid.12