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