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What factors besides underground density affect gravity measurements made at Earth’s surface?

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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.

Beyond underground density distributions, several factors influence gravity measurements taken at Earth’s surface. To isolate the effects of local geology, scientists must account for the following variables:

Gravitational attraction of Earth as a whole: The planet's total mass contributes to the baseline gravitational pull.1
Centrifugal force: This is caused by Earth’s rotation about its axis.21
Elevation: Gravity decreases as distance from Earth’s center increases. This is typically addressed using the free-air correction.1
Surface topography: Unbalanced attractions caused by surface relief require terrain or topographical corrections. Additionally, the mass of rock between the observation point and sea level is accounted for using the Bouguer correction.1
Tidal variations: The gravitational influence of celestial bodies, such as the Moon, causes periodic tidal effects that must be calculated and removed from measurements.13
Latitude: Because Earth is an oblate spheroid rather than a perfect sphere, gravity varies with latitude due to the planet's shape and rotation.21

Properly correcting for these factors is essential for accurate geophysical surveying and understanding the planet's internal structure.41