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How is mean sea level used as an elevation datum?

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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, mean sea level (MSL) serves as a fundamental vertical datum—a reference surface from which elevations are measured. Because the actual sea surface fluctuates constantly due to tides, atmospheric pressure, and wind, MSL is defined as the average height of the sea surface over a long period, typically 19 years or more.12

This average height is established as the "zero elevation" point for vertical control. Surveyors use this datum to determine the heights of surface features through precise methods such as spirit leveling, which involves measuring height differences across the terrain relative to established bench marks.13

While MSL is a primary reference, it is closely related to the geoid, an imaginary level surface that represents the Earth's size and shape based on gravity. In practice, because MSL can deviate from the geoid due to factors like ocean currents, engineering projects often hold MSL at zero elevation to maintain a consistent grid for mapping and construction.1