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Why are elevations on topographic maps measured from a sea-level reference 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.

Elevations on topographic maps are measured from a sea-level reference surface because this surface, known as the geoid, serves as a consistent, physically significant datum for determining height.12

The geoid is defined as an equipotential surface of Earth’s gravitational field that coincides with mean sea level across the oceans. Because the geoid is everywhere perpendicular to the direction of gravity, it provides a natural reference for geodetic leveling, where instruments are set tangential to this equipotential surface.13

While the geoid is not a simple mathematical shape due to irregular mass distributions within the planet, it remains the standard for elevation because it represents the surface to which water would naturally conform. Under continental areas, the geoid is treated as an imaginary continuation of mean sea level. By tying topographic map features to this gravity-based surface, surveyors ensure that elevations are accurately referenced to a consistent physical baseline rather than a purely geometric one.132