Measurement of horizontal angles theodolite by method of repetition

Theodolite: The theodolite is the most intricate and accurate instrument used for measurement of horizontal and vertical angles.

It consists of the telescope by means of which distant objects can be sighted. The telescope has two distinct motions on in the horizontal plane and the Other in the vertical plane.

The former being measured on a graduated Horizontal vertical circle of two verniers.

APPARATUS:-
1.Theodolite
2. Ranging rod
3. pegs etc.

Theodolite is primarily classified as

1) Transit theodolite
2) Non-transit theodolite

A theodolite is called transit theodolite when its telescope can be resolved through a complete revolution about its horizontal axis. In a vertical plane. The transit type is largely used.

Various parts of transit theodolite:-

1) Telescope: it is an integral part and is mounted on the spindle known as the horizontal axis or turn on an axis. The telescope is either internal or external focusing type.

2) The leveling head: It may consist of circular plates called as upper and lower Parallel plates. The lower parallel plate has a central aperture through which a plumb bob may be suspended. The upper parallel plate or tribrach is supported by means of four or three leveling screws by which the instrument may be leveled.

3) To lower plate or screw-plate: It carries horizontal circle at its leveled screw. It carries a lower clamp screw and tangent screw with the help of which it can be fixed accurately in any desired position.

4) The upper plate or vernier plate:- it is attached to the inner axis and carries two verniers and at two extremities diametrically opposite.

5) Compass: the compass box may be either of circular form or of a rough type. The former is mounted on the vernier plate between the standards while the latter is attached to the underside of the scale or lower plate or screwed to one of the standards. Modern theodolite is fitted with a compass of the tubular type and it is screwed to one of the standards.

6) Vertical circle: the vertical circle is rigidly attached to the telescope and moves with it. It is silvered and it is usually divided into four quadrants.

7) Index bar or T-frame: the index bar is T shaped and centered on the horizontal axis of the telescope in front of the vertical axis. It carries two verniers of the extremities of its horizontal arms or limbs called the index arm. The vertical leg called the clip or clipping screws at its lower extremity. The index arm and the clipping arm are together known as T-frame.

Measurement of horizontal angles theodolite by method of repetition

8) Plumb bob: To center the instrument exactly over a station mark, a plumb bob is suspended from the hook fitted to the bottom of the central vertical axis.

Repetition method of measuring Horizontal Angles:-

When it is required to measure horizontal angles with great accuracy as in the case of the traverse, the method of repetition may be adopted. In this method, the same angle is added several times by keeping the vernier to remain clamped each time at the end of each measurement instead of setting it back to zero when sighting at the previous station. The corrected horizontal angle is then obtained by dividing the final reading by the number of repetitions. Usually, six reading, three with face left and three with face right, are taken The average horizontal angle is then calculated.

Procedure:-

1) Let LOM is the horizontal angle to be measured as shown in fig. O is the station point fixed on the ground by a peg? Set up the theodolite over the peg ‘o’ and level it accurately.

2) Set the horizontal graduated circle vernier A to read zero or 360° by upper clamp screw and slow-motion screw. Clamp the telescope to bisect the bottom shoe of the flag fixed at point ‘L’ and tighten the lower clamp. Exactly intersect the center of the bottom shoe by means of lower slow motion screw. Check that the face of the theodolite should be left and the telescope in normal position.

3) Check the reading of the vernier A to see that no slip has occurred. Also, see that the plate levels are in the center of their run. Read the vernier B also.

4) Release the upper clamp screw and turn the theodolite clockwise. Biset the flag bottom shoe fixed at point M by a telescope. Tighten the upper clamp screw and bisect the shoe exactly by means of upper slow motion screw.

5) Note the reading on both the vernier to get the approximate value of the angle LOM.

6) Release the lower clamp screw and rotate the theodolite anticlockwise ai azimuth. Bisect again the bottom shoe of the flag at ‘L’ and tighten the lower clamp screw. By means of slow-motion screw bisect exactly the center of the shoe.

7) Release now the upper clamp screw and rotate the theodolite clockwise. Bisect the bottom shoe of the flag fixed at M and tighten the upper clamp screw. By means of slow-motion screw bisect exactly the center of the shoe. The vernier readings will be now twice the of the angles.

8) Repeat the process until the angle is repeated the required number of times (usually)

9). Add 360° for every complete revaluation to the final reading and divided the total angle by a number of repetitions to get the value of angle LOM.

10) Change the face of the theodolite the telescope will now be inverted. Repeat the whole process exactly in the above manner and obtain the value of angle LOM.

11) The average horizontal angle is then obtained by taking the average of the two angles obtained with face left and face right.

12) Usually, three repetitions face left and three with face right should be taken and the mean angle should be calculated.