Vertical datum data are gathered using which methods?

• A. Tides with sea level measurements, geodetic surveying with different ellipsoid models used with the horizontal datum, and gravity measured with the geoid
• B. Tides alone
• C. Gravity measurements alone
• D. Ellipsoid models used with the horizontal datum

Answer: (A)

Vertical datums are realized by tying elevations to a surface that represents mean sea level, which is connected to the Earth's gravity field. This is done by combining tide measurements to establish mean sea level from coastal and ocean data, gravity data to define the geoid (the gravity-based reference surface close to mean sea level offshore), and geodetic surveying using ellipsoid models to relate the ellipsoidal height system to the vertical datum through appropriate transformations. In short, mean sea level anchors the surface, the geoid provides the gravity-based reference, and the ellipsoid-based horizontal framework is used to ensure the vertical and horizontal datums are consistent. Relying on tides alone lacks a global, stable reference; gravity alone defines the geoid but doesn’t finish the connection to surveyed elevations; and ellipsoid models with the horizontal datum address horizontal positions rather than establishing the vertical reference by themselves.