- To find the elevations of given points with respect to a given or assumed datum (datum is an arbitrarily assumed level surface to which elevations are referred).
- To establish points at a given elevation with respect to a given or assumed datum.
The instruments used in levelling are as follows:
- A level
- A levelling staff
- A measuring tape
MAJOR PARTS OF LEVELLING INSTRUMENT
The level furnishes a horizontal line of sight. The level consists of the following main parts.
- A telescope to provide the line of sight
- A level tube to make the line of sight horizontal
- A levelling head to bring the bubble in its centre of run
- A tripod to support the instrument.
Telescope
Telescope is an optical instrument for magnifying and viewing the images of distant objects. The telescope of a levelling instrument is a metallic tube having an eyepiece at one end and an object glass at the other end. The telescope, which is fitted in levels, is generally of two types.
The external focusing telescopes were used in old model levels and the internal focusing telescopes are being used in modern survey instruments.
External focusing telescope
The telescope in which focusing is achieved by the external movement of either the objective glass or eyepiece is known as an external focusing telescope. In an external focusing telescope, the body is formed by two tubes at the ends of which the objective and eyepiece are fitted. One of the tubes is made to slide axially within the other by means of a rack and pinion arrangement attached to the focusing screw of the telescope (Figure 27.1).
Figure 27.1 An external focusing telescope
Internal focusing telescope
The telescope in which focusing is achieved internally with a concave lens is known as internal focusing telescope. In an internal focusing telescope, the objective and eyepiece are kept at a fixed distance and focusing is achieved by a double concave lens mounted in a short tube capable of sliding axially between the eyepiece and the objective with a rack and pinion arrangement attached to the focusing screw (Figure 27.2).
Figure 27.2 An internal focusing telescope
The objective is a compound lens consisting of a front double convex lens made of crown glass and the back concave-convex lens made of flint glass (Figure 27.3). The two lenses when cemented together with balsam at their common surface are generally known as achromatic lens. In such lenses, the spherical and chromatic aberrations known as optical serious defects are practically eliminated.
The eyepiece is composed of two plane convex lenses with a distance of two-third the local distance. The convex surfaces are turned towards one another. The Ramsden eyepiece is used in most surveying telescopes.
To provide a definite line of sight, horizontal and vertical cross hairs held in a flat metal ring called reticule are fitted into the diaphragm. The diaphragm is a flanged metal ring held in the telescope barrel by four capstan headed screws. With the help of the capstan headed screws, the position of the cross hairs inside the tube can be adjusted slightly, horizontally, vertically and rotationally. The hairs or lines are arranged in different ways as shown in Figure 27.4.
The telescope is used to read the levelling staff and the cross hairs enable the surveyor to take the staff reading.
Types of levelling instrument
The chief types of levels are
- Dumpy level
- Wye (or Y) level
- Reversible level
- Tilting level
Dumpy level
A dumpy level consists of a telescope tube firmly secured in two collars fixed by adjusting screws to the stage carried by a vertical spindle. In the modern form, the telescope tube and the vertical spindle are cast in one piece and a long bubble tube is attached to the top of the telescope. It can neither be rotated about its longitudinal axis nor can be removed from its supports. Figure 27.5 shows the elevation of a dumpy level showing different parts.
Figure 27.3 An objective
Figure 27.4 Different types of cross hairs
In some instruments, a clamp screw is provided to control the movement of the spindle about the vertical axis. For a small or precise movement, a slow motion screw or tangent screw is also provided. The levelling head generally consists of two parallel plates with three foot screws. The upper plate is known as tribrach and the lower plate is known as trivet, which can be screwed on to a tripod (Figure 27.6).
Advantages of a dumpy level
It is simple in construction with a few movable parts. It requires fewer permanent adjustments. Adjustments once carried out remain for a longer period.
Figure 27.5 A dumpy level
Figure 27.6 A dumpy level
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