Levelling instruments

LEVELLING INSTRUMENTS

1. LEVEL

Level is the instrument used for levelling. A level consists of the following parts:

(i) A Telescope for providing the line of sight.

(ii) A Level Tube to make the line of sight horizontal.

(iii) A Levelling Head to bring the bubble of the tube level in its center of run.

(iv) A Tripod for supporting the instrument.

Dumpy Level (Fig. 39)

Dumpy means short and fat (chubby). The dumpy level is a compact instrument with a few movable parts. It needs fewer permanent adjustments than other levelling instruments such as Tilting Level, etc.

A dumpy level consists of a Telescope at the top of which is attached a Level Tube or Bubble Tube. The level tube contains a Spirit Level.

The telescope is provided with an Eye Piece at one end and an Object Glass at the other end. It also contains a Diaphragm, provided with horizontal and vertical hairs. The telescope has a focusing arrangement for changing the focal length of the instrument by adjusting the Focusing Screw. The Levelling Head consists of two parallel plates kept at a distance apart by three Levelling Foot Screws.

Dumpy level is mounted on a Tripod by adjusting screws so that the bubble in the spirit level can be brought central. The line of sight of the telescope, which is fixed parallel to the spirit level, is thus precisely horizontal.

Working Principle

Before taking readings, the following temporary adjustments of Dumpy Level are made:

(i) Setting up the Instrument: The instrument is fixed to the Tripod Stand as shown in Fig. 40. The legs of the tripod are adjusted such that the telescope is approximately horizontal.

(ii) Bringing the Bubble to the Center: The telescope is positioned parallel to two of the three foot screws. Then the telescope is kept perpendicular to the first direction and the third screw is operated to bring the bubble to the center. This process is repeated till the bubble remains at the center for any position of the telescope.

(iii) Eye Piece Adjustment: The eye piece is then focused till the cross-hairs are seen clearly. Now, the instrument is ready for taking readings on the levelling staff.

For the Dumpy Level to be in proper permanent adjustment, the conditions to be fulfilled are:

(a) Line of Collimation is perpendicular to the vertical axis of the Top Rod instrument and (b) Line of Collimation is parallel to the bubble tube axis and is horizontal.

2. TELESCOPIC LEVELLING STAFF (Fig. 41)

Levelling Staff consists of three rectangular rods, namely, Top, Bottom and Central Rods. The central and bottom rods are hollow and the top one is solid. Top rod slides into the central rod telescopically so that the staff is compact when not used.

The levelling staff is made of teak wood. The lengths of levelling staff are 2 m, 3 m, 4 m and 5 m...

Levelling staff is used to measure the height or elevation of the line of sight in meters. Its bottom reads zero. Reading obtained by the line of sight on the staff is the height of the point on which it is held.