Principle of levelling

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.