The theodolite is the most precise instrument used for measurement of horizontal and vertical angles. It can also be used for various surveying operations such as establishing grades, setting out curves, extending survey lines, determining differences in elevation etc.
Two categories classified:
ü Transit theodolite
ü Non-transit thedolite Essential parts of a transit theodolite
A thedolite is provided with a telescope to sight the distant objects clearly. It is mounted on a spindle known as horizontal axis
There are two spindles with axes one inside the other. The outer axis is hallowed and its interior is ground conical to fit the central vertical axis which is a solid and conical.
The outer axis is attached to the lower plate also called the scale plate, having its edge beveled. The edge is silvered and graduated form Oo to 360o in the clockwise direction. The lower plate is provided with a clamped tangent screw or the slow motion screw by means of which it can be fixed at any desired position
The upper plate also called the vernier plate is attached to the inner axis. A clamp and a tangent or slow motion screw are provided for the purpose of accurately fixing the vernier plate to the scale plate. When both plates are clamped together and the lower clamp or loosened, the instrument can be rotated about its outer axis; while if lower plate be clamped and the vernier plate be loosened, the instrument can be rotated about the inner axis. Before either of the tangent screw is turned, the corresponding clamp must be tightened.
Two spirit levels called placed at right angles to each other are fixed on the upper surface of the venires plate for leveling in the instrument.
The vertical circle and graduated and is attached to the horizontal axis of the telescope and thus it rotates with the telescope. The circle is graduated either continuous from Oo to 90o. By means of vertical clamp and tangential screw, the telescope can be set accurately at any position in vertical plane.