At the datum origin where geoid and ellipsoid coincide, the vertical separation between the two surfaces is what value?

• A. Zero
• B. Positive
• C. Undefined
• D. Negative

Answer: (A)

The vertical separation between the geoid and the ellipsoid at a point is the geoid-ellipsoid separation, often called the geoid undulation. It is defined as the distance along the ellipsoid’s normal between the two surfaces. At the datum origin, the two surfaces are defined to coincide, meaning they pass through the same point. Because the ellipsoid and geoid meet there, the distance between them is zero. So the vertical separation at that origin is zero. Outside that origin, the geoid can lie above or below the ellipsoid, giving positive or negative separations, but exactly at the origin it’s zero.