FLY LEVELLING
Fly levelling is a process of finding the level difference
between two points and the levelling consists of taking back sights and fore
sights only and not intermediate sights. Differential levelling is the
determination of level difference between two points; check levelling is finding
the level difference between points in a way of checking the accuracy of
levelling works already done, and fly levelling is the procedure of
accomplishing the objective of finding out the level difference between two
points, which are far apart. It can also be used for transferring a benchmark.
Why we do fly levelling?
Fly levelling procedure
a. Set up the level on a firm ground and do the temporary
adjustments. The instrument should be set up approximately midway between the
change points.
b. Direct the telescope towards the staff, which is held
vertically on the point.
c. Focus the telescope.
d. Bring the staff between the two vertical hairs.
e. Check the bubble. If it is not in the exact centre,
use the screw in line with the telescope and bring it to the centre.
f. Read the staff when the horizontal hair of the
diaphragm appears to cut it and record the correct reading.
g. Take the first reading on the benchmark and enter the
reading in the backsight column of the field book.
h. Take fore sight-reading on the change point, if the
second benchmark is far away, and enter the reading in the foresight column
of the next horizontal line.
i. Shift the instrument and do all the temporary
adjustments.
j. Take back sight on the same change point and enter
the reading in the backsight column of the same horizontal line.
k. Take foresight reading on another change point, if
the next benchmark is far away, and enter the reading in the foresight column
of the next horizontal line.
l. Shift the instrument near to the next benchmark and
again take readings on the change point and enter it in the backsight column
of the same horizontal line.
m. Repeat the above process for a number of benchmarks.
n. If possible close the fly level at the starting point.
o. Record all the reading and data in the field book.
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