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How does the geoid represent Earth’s shape more accurately than a perfect sphere?

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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 represents Earth’s shape more accurately than a perfect sphere because it accounts for the planet's actual gravitational field and mass distribution, rather than assuming a uniform geometric form.12

While a sphere or an ellipsoid of revolution provides a simple mathematical approximation, the geoid is an equipotential surface that is everywhere perpendicular to the pull of gravity. Because Earth’s interior is not homogeneous—containing variations in density and buried-mass concentrations—the force of gravity is not uniform across the planet.134

The geoid coincides with mean sea level over the oceans and extends as an imaginary surface beneath the continents. Due to these gravity anomalies, the geoid is irregular, with its surface deviating by up to 100 meters from a best-fitting reference ellipsoid. By incorporating these physical realities, the geoid serves as a precise reference surface for measuring elevations and understanding Earth's internal structure, such as mantle convection and plate tectonics, which a simple sphere cannot capture.134