Differentiate between each of the following terms used in levelling: i) benchmark and datum surface; ii)level line and horizontal line.(6 marks)June/July 2020

(i) Benchmark and datum surface:

• A benchmark is a fixed point of reference used in surveying to establish a known elevation. It may be a physical mark on a permanent structure, such as a building or bridge, or it may be a mark on a survey monument. Benchmarks are used as reference points for determining the elevation of other points on the ground.
• A datum surface is a reference surface used in surveying to establish a known elevation. It is typically an imaginary plane that is used as a reference point for determining the elevations of other points. The most common datum surface used in surveying is mean sea level.

(ii) Level line and horizontal line:

• A level line is a line that is parallel to the direction of gravity, or perpendicular to the horizon. In surveying, a level line is used as a reference for measuring the elevation of points on the ground.
• A horizontal line is a line that is parallel to the horizon. In surveying, a horizontal line is used as a reference for measuring the position of points on the ground. A horizontal line is not necessarily level, as it may be tilted or sloped relative to the direction of gravity.