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How do ground gravity measurements complement satellite observations when mapping the geoid?

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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.

Satellite observations and ground-based gravity measurements are both essential for mapping the geoid, as they provide complementary data at different scales of resolution.1

Satellites are highly effective at defining the long-wavelength features of Earth’s gravitational field—typically those corresponding to spherical harmonic degrees up to about $l = 16$ , or wavelengths on the order of 2,500 kilometres. However, because the gravitational effects of local mass irregularities attenuate with distance, these orbital observations become less sensitive to smaller-scale features.1

Ground-based gravity measurements are used to fill this gap. By collecting data on the Earth's surface—often reduced to mean values representative of 1° × 1° areas—geophysicists can capture the higher-degree harmonics that satellites cannot resolve. These surface measurements, obtained using gravimeters, are crucial for identifying local density variations caused by geological structures such as salt domes, faults, and ore bodies.12

Ultimately, a comprehensive map of the geoid is produced by synthesizing satellite-derived spherical harmonics with these high-resolution surface gravity anomalies, allowing for a detailed representation of the Earth's equipotential surface.1