Basic Civil & Mechanical Engineering: UNIT II: d. Levelling
Principle of levelling
Surveying
The Principle of Levelling is that a Spirit Level (bubble tube) is to establish a line called the Line of Collimation or Line of Sight.
PRINCIPLE OF LEVELLING
The
Principle of Levelling is that a Spirit Level (bubble tube) is to establish a
line called the Line of Collimation or Line of Sight. The established line lies
in a horizontal plane. It is made to pass through the horizontal crosshair of
the instrument. When the levelling instrument is properly levelled,
(i)
the telescope revolves in a horizontal plane known as Plane of Collimation,
when it is : rotated about the vertical axis;
(ii)
the readings taken on various staff stations are the vertical measurements
below or above the plane of collimation.
See
Fig. 42(a). All the points 1, 2, 3 and 4, where the line of sight strikes, are
in the same horizontal plane.
Further,
the horizontal distances in levelling between the instrument and staff stations
are negligible. Hence, all the points in the horizontal line are taken to lie
on the same level.
See
Fig. 42(b). In levelling practice, there are two important steps:
1.
The levelling instrument is setup at a convenient position. A levelling staff
is held at a point of known elevation (say, B.M. or C.P.). The B.S. is taken on
the staff and the reduced level of line of sight (Height of Instrument H.I.) is
determined.
2.
The staff is then held on points of unknown elevations and staff readings (F.S.
or I.S.) are taken.. The level (R.L.) of the points are computed from the R.L.
of the line of sight (H.I.) and the staff readings.
Height
of Instrument (H.I.) = R.L. of line of sight = R.L. of B.M. + B.S.
R.L.
of a point = H.I. - F.S. or H.I. - I.S.
Note:
1. After every B.S., there must be a F.S. But, there may be many Intermediate
Sights (I.S.).
2.
Levelling must always commence from a permanent B.M. and end on a permanent
B.M.
Basic Civil & Mechanical Engineering: UNIT II: d. Levelling : Tag: : Surveying - Principle of levelling
Basic Civil & Mechanical Engineering: UNIT II: d. Levelling
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Basic Civil and Mechanical Engineering
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