SOME TERMS USED IN LEVELLING
OPERATIONS
OPERATIONS
1. Bench mark (BM): it is a fixed
point on the earth’s surface whose level above ordance
point on the earth’s surface whose level above ordance
datum is known.
2. Ordnance Datum (OD): it is the
mean sea
mean sea
level to which all other levels are
related.
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.
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.
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.
any other sight taken.
6. Reduced Level (RL): calculated
level of a point above or below the datum.
level of a point above or below the datum.
7. Height of Instrument (HI): The
height of the line of collimation above the datum.
height of the line of collimation above the datum.
8. Line of Collimation (LM): It is
an imaginary horizontal line drawn between two points.
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.
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.
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.
dumpy level(automatic level),e.t.c.
. A staff
. Devices for angle measurements
e.g. graphometer, magnetic compass, prismatic compass, orientation compass
e.g. graphometer, magnetic compass, prismatic compass, orientation compass
. Chain or tape
. Pegs, arrows and ranging poles
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