Permanent Adjustment of a Transit Theodolite | Theodolite Surveying | Surveying

The permanent adjustment in case of a transit theodolites are:

1. Adjustment of the Horizontal Plate Levels:

The axis of the horizontal plate levels must be perpendicular to the vertical axis.

2. Collimation Adjustment:

The line of collimation should coincide with the axis of the telescope and axis of the objective slide and should be at right angles to the horizontal axis.

3. Horizontal Axis Adjustment:

The horizontal axis must be perpendicular to the vertical axis.

4. Adjustment of the Telescope Level or Altitude Level:

The axis of the telescope level or altitude level must be parallel to the line of collimation.

5. Vertical Circle Index Adjustment:

The vertical circle vernier must read zero when the line of collimation is horizontal.

For making the adjustments, the instrument should be set up at a fairly level ground where sights of about 100 m can be taken in either direction in the same straight line.

Each adjustment involves two steps:

(i) A test to determine the error, and

(ii) An adjustment of this error.

Since certain adjustments upset others, the adjustments must be made in the order given below:

First Adjustment:

To make the axis of the plate levels perpendicular to the vertical axis.

a. Object:

When this adjustment is made, all the bubbles will remain in the centre of their run during a complete revolution of the instrument.

b. Necessity:

It is important in the measurement of horizontal and vertical angle. The vertical axis should be truly vertical. It may be noted that the error due to vertical axis not being truly vertical cannot be eliminated by taking face left and face right observations.

The adjustment involves making the vertical axis vertical and then making the level on the upper plate perpendicular to the vertical axis.

c. Test:

(i) Set up the instrument on firm ground, clamp the lower motion and turn the upper plate until the longer plate-level is parallel to any pair of foot screws. Bring each plate level to the centre of its run by means of foot screws as explained in temporary adjustment of a theodolite.

(ii) Rotate the instrument about the vertical axis through 180°. The plate level is again parallel to the pair of foot-screws, but with the ends reversed in direction. If the bubble remains central, the axes of the plate- levels are perpendicular to the vertical axis and the vertical axis-is truly vertical.

d. Adjustment:

(i) If not, note down the deviation of the bubble (say, n divisions), which is the apparent error and is twice the actual error in the axis of the level and, therefore,- the correction is half the amount of this error.

(ii) Bring each bubble half way (n/2 divisions) back by means of the respective foot-screw and remaining half way back by means of the captain- headed nuts provided at the end of the tube.

Repeat the test and adjustment until both the bubbles remain central during the whole revolution of the instrument.

Second Adjustment:

To make the line of collimation coincident with the axis of the telescope and axis of the objective slide, and to make it at right angles to the horizontal axis.

This adjustment is made in two steps:

(i) Adjustment of the horizontal hair, and

(ii) Adjustment of the vertical hair.

A. (First Step):

Adjustment of the Horizontal Hair.

a. Object:

The object of this adjustment is to bring the horizontal hair, into the horizontal plain through the optical axis.

b. Necessity:

The movement of the object-glass is assumed to be along the optical axis. If this adjustment does not exist, the direction of the line of sight will change when the objective is moved in and out for focussing. This adjustment is necessary when the instrument is used for measuring vertical angles or when it is used for levelling operations. It has no effect in the measurement of horizontal angles.

c. Test:

(i) Set up the theodolite at a convenient point and level it.

(ii) With the face of the instrument left, take a reading on the staff held on peg B driven at a distance of about 100 m from the instrument station (O). Let the staff reading be b₁. Also note the vertical angle (α).

(iii) Change the face of the instrument. Set the vertical vernier to the former angle (α) and again take a reading on B. If the reading is equal to the previous reading bi, no adjustment is necessary.

d. Adjustment:

(i) If not, note the second staff reading. Let it be b₂.

(ii) Move the horizontal hair by means of the vertical diagram screws until the mean (b) of the two readings b₁ and b₂ is obtained.

B. (Second Step):

Adjustment of the Vertical Hair.

a. Object:

The object of this adjustment is to make the line of collimation perpendicular to the horizontal axis.

b. Necessity:

If this adjustment exists, the line of collimation will generate a plane when the telescope is transited. But if not, it will generate a cone with the horizontal axis as its axis. The adjustment is necessary for prolonging straight lines and for measuring horizontal angles between points at different elevations.

c. Test:

(i) Set up the theodolite at a convenient point O on a fairly level ground and level it. Fix a peg at a point A at a distance of about 100 m from O. Bisect A and clamp the lower and upper screws.

(ii) Now transit the telescope and mark a point B in the line of sight at about 100 m from O and at about the same level as A.

(iii) Unclamp the upper screw, swing the telescope through 180°, and again bisect A. Then clamp the upper screw.

(iv) Transit the telescope again. If now the point B is again bisected by the cross-hairs, the adjustment is correct.

d. Adjustment:

(i) If the point B is now not on the line of sight, mark a point C in the line of sight opposite B. The apparent error BC is four times the real error, since the telescope is transited twice.

(ii) Mark a point D at one fourth distance from C to B (CD = 1/2CB).

(iii) Move the diagram by means of the horizontal diagram screws until the vertical hair is on the point D.

Repeat the process until the adjustment is correct.

Third Adjustment:

To make the horizontal axis perpendicular to the vertical axis.

a. Object:

The object of this adjustment is to ensure that the line of sight revolves in a vertical plane perpendicular to the horizontal axis.

b. Necessity:

The adjustment is necessary for prolonging straight lines and for measuring horizontal angles.

The above condition can be established by the Spire Test as follows:

c. Test:

(i) Set up the instrument near a spire or some other elevated object. Let P be the top point.

(ii) Sight the point P, and with both horizontal motions clamped, depress the telescope and mark a point A in the line of sight near the base of the object.

(iii) Change the face of the instrument, and again sight on the point P. Depress the telescope. If the line of sight now strikes the point A marked previously, the adjustment is correct.

d. Adjustment:

(i) If not, mark another point B in the line of sight near the base of the same object. Mark another point C midway between A and B.

(ii) Sight on point C and clamp the upper motion. Raise the telescope. The line of sight will not now strike the point P. Raise or lower the adjustable end of the horizontal axis by means of the screws near the top of the A-frame until the line of sight passes through the point P.

Repeat the test the adjustment until perfect.

Fourth Adjustment:

To make the axis of the telescope level or altitude level parallel to the line of collimation.

a. Object:

The object of this adjustment is that the line of collimation should remain horizontal when the telescope level is brought in the centre.

b. Necessity:

The adjustment is essential when the vertical angles are to be measured and the instrument is to be used as a level.

c. Test:

The procedure of testing the theodolite for this adjustment is the same as in the ‘two peg’ method of the dumpy level.

(i) Fig. 9.29:

Fix two pegs A and B on a fairly level ground about 100m apart. Set up the theodolite at O exactly midway between A and B. Clamp the vertical circle and the telescope level in the centre of its mm by means of the tangent screw of the vertical circle.

(ii) With the bubble central, take readings on staff held on A and B and find the difference between these readings, which is the true difference of level between A and B.

(iii) (Fig. 9.30.) Shift the instrument and set it up at O₁ on the line BA produced, at about 20 m from A.

(iv) With the bubble central, read the staff first on A and then on B and find the difference between the two readings. If this difference is equal to the first difference, the adjustment is correct.

d. Adjustment:

(i) If not, calculate the correct staff readings on A and B as explained in the ‘two-peg’ adjustment of the dumpy level.

(ii) Bring the horizontal hair exactly to the correct reading on B by means of the tangent screw of the vertical circle. The bubble is thus disturbed.

(iii) Bring the bubble exactly to the centre of its run be means of the level tube nuts.

(iv) Sight the staff on the near peg and note whether the calculated correct reading is obtained.

Repeat the process until the adjustment is correct.

Fifth Adjustment:

To make the vertical circle read zero when the line of collimation is horizontal i.e. the telescope bubble is centered.

a. Object:

The object of this adjustment is that the vertical circle should read zero when the line of collimation is horizontal.

b. Necessity:

This adjustment made for convenience only. If the index error i.e. the reading on the vertical circle when the telescope bubble is in the centre, is noted and the corresponding correction applied to the observed reading, no error will be introduced. But still this adjustment is preferred while reading vertical angles and prolonging straight lines.

c. Test:

(i) Set up the theodolite at firm ground.

(ii) Centre the plate bubbles and then bring the telescope bubble exactly to the centre of its run by means of the vertical tangent screw as in the first adjustment, and read the vernier of the vertical circle. If it reads zero, then this adjustment of the instrument is correct.

d. Adjustment:

If the vernier does not read zero, loosen it and move it by means of the screw which holds it to the standard until if reads zero.