Basic principles of theodolite traversing

Basic principles of theodolite traversing Theodolite is an instrument designed for the measurement of the horizontal and vertical angle.

 It is most precise method it is also used for laying of horizontal angles Locating points on line prolonging the survey line establishing the gradient, determination of the difference in the elevation setting out a curve.

in transit, theodolite telescope can be revolved a complete revolution about its horizontal axis in a vertical plane. a transit theodolite consists of the essential part.

Leveling head

It supports the main working parts of the instrument and screws on a tripod. The head comprises of two parts a) A leveling foot screws for leveling the instrument i.e. for marking vertical axis truly vertical. A movable head or centering arrangement for centering the vertical axis accurately over a station point.

A lower level circular horizontal metal plate

It carries a circular graduated arc. The lower plate is attached to a vertical metal spindle (outer axis) which works in vertical bearing and a form a part of leveling head. It may be graduated in degree and a half degree or a degree 1/3 of degrees .the upper plate carries an index and vernier or micrometer towards fine reading on graduated horizontal circle .the upper plate carries standard use of for supporting the telescope and the spirit level used for leveling the instrument.

A telescope

The telescope is pivoted between the standard at right angles to the telescope is provided with the focusing screw, Clamping screw and tangent screw.

A circular graduated are carried on vertical circle

It is attached to the horizontal axis of the telescope, it is usually divided into 4 quadrants, but in some instruments, it is graduated continuously from 0-3600. the graduation in each quadrant is numbered from 0-900 in opposite direction. The subdivisions of the vertical circle are similar to those of vertical circle. MEASUREMENT OF VERTICAL ANGLE A vertical angle is an angle between the inclined line of sight to an object and the horizontal. It may be an angle of elevation or on the angle of depression according to as the point is above or below the horizontal plane passing through the trunnion axis of the instrument. To measure the angle of elevation or depression LOM shown in fig. proceed as. follows:

 1) Set up the theodolite at station point O and level it accurately with reference to the altitude level. 2) Set vertical verniers C and D exactly to zero by using the vertical circle clamp and tangent screw, while the altitude level should remain in the center of its run. Also, the face of the theodolite should be left. 

3) Release the vertical circle clamp screw and rotate the telescope in a vertical plane so as to bisect the object M. tighten the vertical circle clamp and exactly bisect the object by a slow-motion screw. 4) Read both verniers C and D. the mean of the two readings gives the value of the required angle. 

5) A similar observation may be made with another face. The average of the two values thus obtained gives the value of the required angle which is free from instrumental errors.

 6) Similarly, the angle of depression can be measured following the above steps. To measure the vertical angle between two points L and M Sometimes it is required to measure the vertical angle between two points L and M. There can be three possibilities. 

(a) One point is above the line of sight and the other is below the line of sight then angle LOM as shown in fig will be equal to (<α +<β ) 

(b) Both the points are above the line of sight. Then the angle LOM= <α -<β (Refer Fig 2)

 (c) Both the points are below the line of sight, then the angle LOM= <α -<β (Refer Fig 3) To measure the angle between two points L and M proceed as follows 

1) Set the theodolite at station point O and accurately level it. 

2) Bisect the flag at L as explained already and take the reading on the verniers C and D. Calculate the mean angle.

basic principles of theodolite traversing