Levelling terminology: A relatively fixed point whose elevation above a stated datum is known (or established) and used for transferring levels is called what?

Introduction / Context:
Successful levelling needs reliable reference elevations. Surveyors therefore establish and maintain permanent points with known elevations. The question asks for the precise term used in field books and specifications for such points.

Given Data / Assumptions:

• A known elevation is tied to a consistent datum (e.g., mean sea level or project datum).
• The point is physically stable and recoverable.
• It is intended for repeated use to start or check levelling runs.

Concept / Approach:
The correct term is “bench mark” (BM). A BM’s reduced level (RL) is known; the RL is a numerical value, not the point itself. “Datum” is the zero or reference level, not the mark. “Reference point” is generic and may lack a known elevation. Hence, “bench mark” uniquely identifies the object described.

Step-by-Step Solution:

Verification / Alternative check:
Survey manuals classify BMs as GTS (Great Trigonometrical Survey), permanent BMs, arbitrary BMs, and temporary BMs, all serving the same purpose of carrying elevations.

Why Other Options Are Wrong:

• Datum point: ambiguous and usually refers to the abstract reference level, not a field monument.
• Reduced level: the numeric elevation value, not the point.
• Reference point: may be geometric only; not necessarily with known elevation.

Common Pitfalls:
Writing “RL of BM” backward as “BM of RL”; failing to protect or describe BMs properly leading to loss; assuming a temporary mark’s stability without re-check.

Final Answer:
Bench mark