The theodolite is most accurate instrument used mainly for measuring horizontal and vertical angles. It can also be used for locating points on a line, prolonging survey lines, finding difference in elevations, setting out grades, ranging curves etc.
Parts of Theodolite and their Functions
A transit vernier theodolite essentially consists of the following parts:
- Leveling Head
This supports the main working parts of the instrument and screws on to a tripod. It comprises of two parts: (a) Tribrach and trivet stage fitted with leveling screws, (b) Center shifting arrangement for centering the instrument quickly and accurately.
- Lower Circular Plate
This carries the circular scale graduated from 0° to 360° and half degrees and third of a degree, and a tapered spindle which works in the outer conical bearing. To the spindle is fitted a well ribbed bracket carrying on opposite side the slow motion and clamping screws for upper plate and lower plate.
- Upper Plate
The center of the vertical spindle of the lower plate is bored from a bearing for an other vertical spindle which carries the upper circular horizontal plate. The upper plate can be rotated relative to the lower plate about this spindle as axis. It carries two verniers marked A and B, which are used for taking readings accurately up to 20 degree on the lower graduated circle. This plate also carries a level tube and two vertical standards for supporting telescope, vertical circle and detachable compass. The compass may be:-
(a) the circular box compass.
(b) the trough compass.
(c) the tubular compass.
The telescope of theodolite may be:-
(a) external focusing.
(b) internal focusing. The first type is used in older type of theodolites, while the later is used in modern instruments. It is mounted near its center on a horizontal axis at right angles to the main longitudinal axis of the telescope.
- Vertical Circle
The vertical circle is rigidly fixed on the horizontal axis of the telescope and more with it. It is silvered and is usually divided into four quadrants. The graduations in each quadrant are numbered from 0 degree to 90 degree in opposite directions from the two zeros placed at ends of the horizontal diameter of the vertical circle so that the line joining the zeros is parallel to the line of collimation of the telescope when it is horizontal. The sub-divisions on the vertical circle are similar to those of horizontal circle. A clamp and fine motion tangent screws are provided for the vertical circle.
- T-Frame or Index Bar
It is T-shaped and is centered on the horizontal axis of the telescope in front of the vertical circle. The two verniers C and D are provided on it at the ends of the horizontal arms called the index arm. A vertical leg, known as clipping arm is provided with a fork and two clipping screws at its lower extremity. The index and clipping arms together are known as T-frame. At the top of this frame is attached a bubble tube which is called the altitude bubble tube.
A plumb-bob is suspended from the hook fitted to the bottom of the vertical axis for centering the instrument exactly over a station point.
- Tripod stand
The theodolite is supported on the tripod when in use.