Levelling:

· It is a surveying method used to determine the level of points/objects with reference to the selected datum.

· It is also used to set out engineering works.

Uses of Levelling:

· To determine the difference in levels of points/Objects

· To obtain contour map of an area

· To obtain cross section of roads, canals etc.,

· To determine the depth cutting and filling in engineering works.

· To establish points or erect machinery or construct a building component at a predetermined level.

Important Terms:

Bench Mark:

It is surveyor’s mark cut on a stone/ rock or any reference point used to indicate a level in a levelling survey.

Reduced Level:

· Reduced level of a point is the level of the point with respect to the level of permanent feature or bench mark.

· It indicates whether the point is above or below the reference point (datum).

Instruments used in levelling:

Instruments used in levelling are,

(i) Levelling instrument

(ii) Levelling staff

Levelling Instrument :

· Simplest form of levelling instrument is dumpy level.

· The different parts of levelling instrument are,

(a) Telescope (b) Eye-piece (c) focussing knob (d) level tube (e) cross bubble (f) foot screws (g) levelling head (h) diaphragm (i) ray shade

Dumpy Level:

Levelling Staff:

· It is an important accessory used with levelling instrument at the time of conducting levelling survey.

· Reading is taken on the levelling staff held properly at the point concerned by viewing through the telescope of the levelling instrument.

· Usually 4 m levelling staff may be used of folding type or telescopic type

· Aluminium levelling staff foldable at every metre length has also came to the market

· The levelling staff consists of three pieces.

· The topmost one slides into the middle one and the middle portion slides into the bottom one.

· When the staff is fully pulled, it will read exactly 40 decimeters (4m) from the bottom shoe.

Graduation in levelling staff:

· Every metre length is divided into 200 divisions.

· The divisions are painted in black and white alternately of thickness 5 mm each.

· The graduation figures are marked at every decimeter length.

· The number indicating metre is in red and the decimeter number is in black

· Thus, a graduation figure of 24 indicates 2 metres and 4 decimeters.

· The graduation are made continuously one above the other in the same line.

· The division lines should be parallel to the base of the bottom shoe and perpendicular to the length of the staff.

· The edges of the division lines should be straight sharply defined.

· They should be clear and made distinctly visible by properly contrasting.

· The graduation colour paints used should not crack or blister when exposed to adverse or atmospheric conditions.