Define each of the following terms as used in levelling: (i) back sight; (ii) vertical line; (iii)reduced level; (iv) temporary benchmark.(6 marks)June/July 2020

(i) Back sight: A back sight is a measurement taken with a leveling instrument, such as a transit or theodolite, that establishes the starting point for a series of level readings. It is taken by aiming the instrument at a known point of reference, such as a benchmark or previous sight point, and measuring the vertical angle to that point.

(ii) Vertical line: A vertical line is a line that is perpendicular to the horizon, or parallel to the direction of gravity. In levelling, a vertical line is established by sighting through a leveling instrument and adjusting the instrument until it is level.

(iii) Reduced level: Reduced level, also known as relative level, is the vertical distance between a point on the ground and a reference level, such as mean sea level. In levelling, reduced levels are determined by taking a series of level readings from a known starting point and using them to calculate the elevations of other points on the ground.

(iv) Temporary benchmark: A temporary benchmark is a temporary reference point used in levelling to establish a known elevation. It may be a small stake or rod driven into the ground, or a mark on a wall or other permanent structure. Temporary benchmarks are typically used in situations where a permanent benchmark is not available or is not suitable for use.