LESSON NOTE ON SOME TERMS USED IN LEVELLING OPERATIONS

SOME TERMS USED IN LEVELLING OPERATIONS

1. Bench mark (BM): it is a fixed point on the earth’s surface whose level above ordance

datum is known.

2. Ordnance Datum (OD): it is the mean sea

level to which all other levels are related.

3. Back sight (BS): is the first sight taken after the level has been set up. A sight taken to a point whose height is known or can be calculated.

4. Foresight (FS): The last sight taken. A sight taken to a point whose height is required to carry on the line of level.

5. Intermediate Sight (IS): it is any other sight taken.

6. Reduced Level (RL): calculated level of a point above or below the datum.

7. Height of Instrument (HI): The height of the line of collimation above the datum.

8. Line of Collimation (LM): It is an imaginary horizontal line drawn between two points.

9. Rise and fall: The difference is height or is level between two is referred to as a rise or fall.

10. Change Point (CP): the point at which both a foresight and then a back sight are taken.

LEVELING INSTRUMENTS

These include:

. A Level e.g. theodolite, transit dumpy level(automatic level),e.t.c.

. A staff

. Devices for angle measurements e.g. graphometer, magnetic compass, prismatic compass, orientation compass

. Chain or tape

. Pegs, arrows and ranging poles