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Why is a spirit level set tangent to an equipotential surface during surveying?

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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.

In surveying, a spirit level is set "level," or tangential to an equipotential surface, to determine the elevations of points on the land surface through geodetic leveling.1

Because the geoid is defined as an equipotential surface of Earth's gravitational field, setting the instrument tangential to this surface ensures that measurements are taken with respect to the equipotential and, consequently, very nearly with respect to the geoid itself. This process allows surveyors to establish accurate height measurements relative to a bench mark, which is essential for mapping and civil engineering projects.123