When the geoid coincides with the ellipsoid at the datum origin, the normal to the ellipsoid is aligned with which line at the datum origin?

• A. Plumb line
• B. Geoid line
• C. Ellipsoid axis
• D. Equatorial line

Answer: (A)

The local vertical is what’s being represented here. When the geoid and the ellipsoid touch at the datum origin, the ellipsoid’s normal there points in the same direction as the gravity vector. That direction is the plumb line, i.e., the local vertical. So the normal to the ellipsoid at the origin aligns with the plumb line.

(The other ideas—like the ellipsoid’s symmetry axis or a term for a “geoid line” or an equatorial line—do not represent the gravity direction at that point.)