Which statement about the geoid is true?

• A. It coincides with the surface oceans would conform to when free to adjust to gravity and rotation
• B. It is identical to the reference ellipsoid
• C. It is a perfect sphere
• D. It is the mean sea floor topography

Answer: (A)

The geoid is an equipotential surface of Earth's gravity field that would coincide with mean sea level if the oceans were free to adjust to gravity and rotation. In other words, it’s the shape the ocean surface would take globally under the influence of gravity and the rotation of the Earth, with landmasses removed. This is why the geoid follows the irregularities of Earth's gravity: variations in mass inside the planet pull the surface up or down locally.

This is why the geoid is not the same as the reference ellipsoid, which is a smooth, mathematically convenient figure used for mapping. The geoid has bumps and hollows determined by gravity anomalies, so it can rise or fall by tens of meters relative to the ellipsoid. It is also not a perfect sphere, since Earth’s mass distribution is uneven and the rotation causes a slight equatorial bulge. And it is not the mean sea floor topography, which lies far below the surface and is shaped by tectonics and sedimentation, not by the gravity-driven equipotential surface that defines the geoid.