Which statement correctly differentiates vertical datum from a height system?

• A. A vertical datum defines zero height reference; a height system provides height values relative to that datum (e.g., orthometric height).
• B. A vertical datum is the same as a height system; both define absolute height above sea level.
• C. A vertical datum is used only for ocean depths; a height system is for land elevations.
• D. A vertical datum includes gravity measurements; a height system includes spirit leveling.

Answer: (B)

Vertical positioning rests on a defined zero surface and a way to read heights from that zero. A vertical datum names the reference surface used for vertical measurements. A height system is the method that assigns numeric height values relative to that reference (for example, orthometric height uses the reference surface tied to mean sea level or a geoid). When the datum is anchored to mean sea level, the height values produced by the height system are effectively heights above sea level. In this framing, the vertical datum and the height system are two parts of the same framework: the datum provides the zero, and the height system provides the values. The other ideas—treating them as entirely different concepts focused on different domains, or tying one only to depths versus elevations, or relying solely on gravity measurements or spirit leveling—don’t capture how these components work together to express an absolute vertical position.