Which statement best defines a vertical datum?

• A. Measures the elevations of specific points on the earth.
• B. Is a collection of specific points on the Earth with known heights either above or below mean sea level (MSL).
• C. This data is gathered via tides with sea level measurements, geodetic surveying with different ellipsoid models used with the horizontal datum, and gravity, measured with the geoid.
• D. A method used to measure atmospheric pressure at survey sites.

Answer: (B)

A vertical datum is the reference surface used to define and measure elevations, providing a zero-height baseline for all vertical measurements. It is typically realized as a network of known points (benchmarks) whose elevations are defined relative to a reference surface, such as mean sea level or a geoid. Saying it as a collection of specific points with known heights above or below mean sea level captures this idea: those points establish the vertical reference so every height can be measured against the same standard. The other descriptions describe either performing elevation measurements, mixing concepts about how vertical references relate to horizontal references and gravity, or refer to atmospheric pressure, none of which define what a vertical datum is.