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How does geodetic leveling measure elevation relative to 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.

Geodetic leveling determines the elevation of points on the Earth's surface by using a spirit level to establish a line that is "level," or tangential to an equipotential surface of the Earth's gravitational field.1

Because the geoid is defined as the specific equipotential surface that coincides with mean sea level across the oceans, measurements taken relative to these level surfaces are effectively measured with respect to the geoid. During the process, surveyors take sights on calibrated rods to determine the differences in elevation between points.1

While these measurements provide high accuracy, they are subject to slight distortions due to variations in gravity and centrifugal force at different latitudes. Consequently, for long-distance leveling, especially along north–south lines at high altitudes, orthometric corrections are applied to ensure the final elevations accurately represent the height above the geoid.2