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How does the geoid relate to mean sea level over the oceans?

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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 defined as the specific equipotential surface of Earth’s gravitational field that coincides with the unperturbed mean sea level over the oceans.12

In this context, "unperturbed" means that the effects of dynamic factors—such as waves, tides, winds, and ocean currents—are removed. Because the geoid is everywhere perpendicular to the pull of gravity, it reflects variations in Earth's gravitational field caused by internal density heterogeneities and seafloor topography.132

While the actual ocean surface is warped by these gravitational influences—for example, bulging over massive seamounts or downwarping over deep-sea trenches—the geoid represents the theoretical surface to which the ocean would conform if it were at rest. Consequently, the geoid serves as a vital reference surface for geodesy, allowing scientists to measure precise elevations and map the Earth's gravity field.1423