When geoid and ellipsoid coincide at a datum origin, the deflection and the ______ and the separation between the ellipsoid and geoid become ______ at the origin.

• A. Horizontal, zero
• B. Vertical, zero
• C. Vertical, positive
• D. Horizontal, positive

Answer: (B)

At the datum origin where the geoid and ellipsoid meet, the two surfaces share the same tangent plane, so the local vertical defined by gravity aligns with the ellipsoid’s normal. This makes the deflection of the vertical zero, and the separation between the ellipsoid and geoid along that vertical also zero, because the surfaces pass through the same point there. Away from the origin, the two surfaces can diverge, giving nonzero deflection and a nonzero geoid–ellipsoid separation.