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Why are orthometric corrections needed for some long leveling lines?

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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 surveying, orthometric corrections are necessary for long leveling lines because the Earth’s level surfaces are not perfectly parallel.1

These surfaces, which are defined by the gravity field, become slightly distorted as one moves toward the Earth's poles due to the increase in gravitational force and the corresponding reduction in centrifugal force at higher latitudes. Because of this convergence, the physical distance between these level surfaces and the geoid does not remain constant.1

Consequently, for long leveling lines—particularly those with a significant north–south trend—the measured differences in elevation do not exactly represent the true heights above the geoid. Applying an orthometric correction accounts for these distortions, ensuring that the vertical control data remains accurate across large geographic distances.1