What is used to mark elevation in surveying?

The correct answer is datum. In surveying, a datum serves as a reference point or a baseline from which measurements of elevation are taken. This reference point is critical because it establishes a common standard for planners and builders to measure heights and depths accurately across different locations.

Datums can be local or global and provide a consistent framework for understanding how elevations relate to one another. For instance, a common datum used in the United States is the North American Vertical Datum of 1988 (NAVD88), which provides a consistent basis for evaluating elevations across the country.

Other choices might refer to different aspects of surveying. A benchmark, for example, is a specific point that has a known elevation but is often used in conjunction with a datum. A level line is related to horizontal measurements rather than elevation, and an altitude marker is typically not a standard term used in surveying for establishing elevation. Thus, datum is the most accurate term that encapsulates the reference system for measuring elevation.