What does the geoid represent within geodesy?

• A. The equatorial plane.
• B. The Earth's center.
• C. The reference ellipsoid.
• D. An equipotential surface approximating mean sea level used as reference for orthometric height.

Answer: (D)

The geoid is the gravity-based reference surface that corresponds to mean sea level. It is an equipotential surface, meaning gravity has the same potential everywhere on it, so a vertical plumb line would be perpendicular to it. Because Earth’s gravity varies with density, topography, and rotation, this surface is irregular and undulates rather than being a perfect sphere or ellipsoid, extending beneath the continents as well as the oceans. Surveying and geodesy use the geoid as the vertical datum for orthometric height—heights measured above this surface—unlike the smooth reference ellipsoid, which is a mathematical model of Earth’s shape, or the equatorial plane, or Earth’s center.