Which of the following best describes the geoid?

• A. Ellipsoid height is measured from the geoid.
• B. The geoid is the distance above the reference ellipsoid.
• C. The geoid is the undulating surface of equal gravitational potential that approximates mean sea level.
• D. Geoid height is used for navigation at sea only.

Answer: (C)

The geoid is the surface of equal gravitational potential that would follow mean sea level if the oceans could flow freely around the globe, ignoring winds and tides. It undulates because Earth's gravity field varies with mass distribution, so the geoid is irregular rather than a smooth shape. This surface provides the reference for elevations, with geoid height describing the separation between the geoid and a smooth reference ellipsoid, a key concept for converting GPS ellipsoidal heights to true heights above mean sea level. The statement that best describes the geoid is that it is an undulating surface of equal gravitational potential approximating mean sea level. The other ideas mix up what the geoid represents or what height measurements refer to, since ellipsoid height is measured relative to the ellipsoid, not from the geoid, and geoid height is not limited to navigation at sea.