LEVELLING

LEVELLING

WHAT IS LEVELLING?

Levelling is an operation in surveying performed to determine thedifference in levels of two points.By this operation the height of a point froma datum, known as elevation, is determined.

DEFINITIONS

LEVEL SURFACE

A level surface is the equipotential surface of the earth’s gravity field. It is acurved surface andevery element of which is normal to the plumb line.

LEVEL LINE

Any line laying on a levelled surface is called a level line. This line is normal to the plumb line (direction of gravity) at all points (Fig. - L.1).

HORIZONTAL PLANE

Any plane tangential to the level surface at any point is known as the ‘horizontal plane’. It is perpendicular to the plumb line which is indicates the direction of gravity.

HORIZONTALLINE

Any line laying on the horizontal plane is said to be a horizontal line. It is a straight line tangential to the level line (Fig. - L.1).

VERTICALLINE

The direction indicated by a plumb line (direction of gravity) is known as the vertical line (Fig. L.1).

VERTICALLINE

Any line passing through the vertical line is known as the vertical plane.

DATUM

A datum is a reference surface of constant potential, called as a level surface of the earth’s gravityfield, for measuring the elevations of the points. One of such surfaces is the mean sea level surfaceand is considered as a standarddatum. Also, an arbitrary surface may be adopted as a datum.

Reduced level (R.L.):

Reduced level of a point is its height or depth above or below theassumed datum. It is the elevation of the point.

Line of Collimation:

  It is an imaginary line passing through the intersection of cross-hairs at the diaphragm and the optical centre of the object glass and its continuation. It is also known as line of sight.

Axis of Telescope:

  This axis is an imaginary line passing through optical centre of the object glass and the optical centre of the eye-piece.

Bench Mark (BM):

  This are fixed points or marks of known RL determined with reference to the dutam line. These are very important marks. They sever as reference points for finding RL of new points or for conducting levelling operation in projects involving roads, railways, etc.

  Bench-marks may be of four types in India [a] GTS, [b] permanent, [c] temporary, and [d] arbitrary.

[a] GTS Bench-marks – These bench-marks are established by Survey of India Department at a large interval all over the country. The value of reduce levels, the relevant position and the number of bench-marks are published by the department.

[b] Permanent Bench-marks – These are fixed points or marks established by Government department like PWD, Railways, Irrigation, etc. The RLs of these points are determined with the reference to the GTS bench-mark and are kept on permanent points like the plinth of a building, parapet of bridge or culvert, so on. Sometimes they are kept on underground pillars.

[c] Temporary Bench-marks – When the bench-marks are established temporarily at the end of day’s work; they are said to be temporary bench-marks. They are generally made on root of tree, the parapet of nearby culvert, a furlong post, or on a similar place.

[d] Arbitrary Bench-marks – When the RLs of some fixed points are assumed, they are termed arbitrary bench-marks. These are adopted in small survey operations when only the undulation of the ground surface is required to be determined.

Back sight (B.S.):

It is the first reading taken on the staff after setting up the level usuallyto determine the height of instrument. It is usually made to some form of a bench mark (B.M.) orto the points whose elevations have already been determined. When the instrument position mustbe changed, the first sight taken in the next section is also a back sight(Fig. - L.2).

Fore sight (F.S.):

It is the last reading from an instrument position on to a staff held at a point.It is thus the last reading taken within a section of levels before shifting the instrument to the nextsection, and also the last reading taken over the whole series of levels (Fig. - L.2).

Change point (C.P.) or turning point:

A change point or turning point is the point whereboth the fore sight and back sight are made on a staff held at that point. A change point is requiredbefore moving the level from one section to another section. By taking the fore sight the elevationof the change point is determined and by taking the back sight the height of instrument is determined.The change points relate the various sections by making fore sight and back sight at the same point (Fig. - L.2).

Height of instrument (H.I.) or height of collimation:

For any set up of the level, theelevation of the line of sight is the height of instrument (Fig. - L.2).

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