In vertical deflection, the actual plumbline is perpendicular to the geoid. The normal plumbline is perpendicular to the ellipsoid. Which surface is the actual plumbline perpendicular to?

• A. Geoid
• B. Ellipsoid
• C. Sphere
• D. Horizon

Answer: (A)

Vertical direction follows local gravity. The plumbline itself is the line of action of gravity at a point, and gravity is perpendicular to an equipotential surface. The geoid is the equipotential surface that best represents mean sea level, so the actual plumbline is perpendicular to the geoid. The normal plumbline to the reference ellipsoid is a construction used for the mathematical reference surface, and the angle between the actual plumbline and that ellipsoid normal is the vertical deflection. The sphere or horizon aren’t the surfaces guiding the true gravity here.