Level

Several types of levelling instruments used can be listed as follows

(a)        Dumpy level

(b)       The Wye or Y level

(c)        Cooke's reversible level

(d)       Cushing's level

(e)        Tilting level and

(f)        Automatic level

Dumpy Level is by far the most commonly used level in engineering surveys and hence described in detail here. Only important features of other types of levels are described.

Dumpy level is a stable, simple, and compact instrument along with various components as shown in Figure.

                                                    Figure : Dumpy Level

The telescope is rigidly attached to supports with a longitudinal bubble tube fixed at its top. A telescopic tube cannot be rotated about its horizontal axis, nor can it be removed from the supports. A transverse bubble tube at right angles to main bubble tube is given to adjust the level's plane in horizontal position. The telescope consists of object glass, eye piece and a circular ring diaphragm along with cross wires. The telescope commonly has a magnification factor of thirty. The eye piece could be rotated to bring crosswire of the diaphragm distinct and clear. Focusing screw helps in bringing the object's unique image within the diaphragm plane.

The levelling head is given with two rigid parallel plates along with three screws to level the instrument. Within a few instruments, a compass is also given below the telescope to measure bearings. Since the dumpy level has only a few movable parts, it is easily to be set and adjusted that are not easily disturbed providing sturdiness to the instrument.

Y levels are sensitive and delicate instruments. The telescope is removable and could be reversed end to end. This gives the facility of making indoor adjustment easily and rapidly.

Within Cushing's level, though the telescope can't be removed, the eye piece along with diaphragm and the object glass are removable and hence can be interchanged.

The Tilting levels are used for precision levelling. The telescope can have a little motion about a horizontal axis only below the telescope axis.

Automatic levels are also termed as self aligning levels. The levelling of instrument is automatic as against manual levelling through spirit bubbles in conventional stages.