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How is the geoid extended beneath continents where sea level cannot be observed directly?

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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 an equipotential surface that coincides with mean sea level over the oceans. Because this surface is not directly observable beneath landmasses, it is conceptually extended under continents as the imaginary surface to which water would rise if narrow canals were cut through the land from ocean to ocean.123

Historically, this surface was defined in continental areas using spirit leveling, where measurements are taken relative to an equipotential surface. In modern geodesy, the global geoid is determined by synthesizing spherical harmonics derived from gravity anomalies and the orbital perturbations of artificial satellites. By combining these satellite observations with surface gravity measurements, scientists can map the geoid's undulations across both oceanic and continental regions.1243