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How does the geoid serve as a reference surface in surveying?

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surveying Figure 1: Photogrammetric photographs from two short, overlapping flight strips arranged for supplying mapping details. Photo-control points are shown on only one photograph; shading indicates a typical terrain feature such as a lake (see text).

In civil engineering and geodesy, the geoid serves as a critical reference surface for determining precise elevations of Earth's surface features.1

Because the geoid is an equipotential surface—meaning the potential of gravity is constant across its entire extent—it is everywhere perpendicular to the direction of gravity. In practice, surveyors use spirit levels set tangential to this equipotential surface to measure height differences, effectively referencing elevations to the geoid.21

While the geoid accurately represents mean sea level over the oceans and an imaginary sea-level surface under continents, its irregular shape caused by mass distribution makes it unsuitable as a simple geometric reference for horizontal positioning. Consequently, engineers often use a regular reference ellipsoid for geometric calculations, while the geoid remains the essential datum for defining vertical height above sea level.31