What is the geoid?

• A. The shape of the oceans under gravity and rotation alone
• B. A perfectly smooth ellipsoid
• C. The mean elevation surface of continents
• D. A hypothetical flat plane

Answer: (A)

The geoid is the equipotential surface of Earth's gravity field that would coincide with mean sea level, extended under the continents. Picture the oceans in perfect hydrostatic balance, affected only by gravity and the Earth’s rotation—the surface they would form is the geoid. Because gravity varies with Earth’s internal structure, the geoid is irregular rather than a smooth shape, with undulations that reflect gravitational anomalies.

This concept matters in surveying because GPS provides ellipsoidal heights, which must be related to elevations above sea level. The difference between the geoid and the reference ellipsoid (the geoid undulation) is what you use to convert ellipsoidal heights to orthometric heights.

The other options don’t fit: a perfectly smooth ellipsoid is just a simple mathematical model, not the irregular geoid; the mean elevation surface of continents is a topographic crustal surface, not the gravity-based equipotential surface; and a flat plane is incorrect because the geoid varies with gravity and rotation.