Question

Describe the temporary adjustment of theodolite

Answer 1

Temporary Adjustments:

The temporary adjustments have to be carried out at every setup of the instrument before taking observations with the theodolite

A)Setting up:

1) Place the instrument over the station by spreading the tripod legs well apart at a convenient height.

2) Suspend a plumb bob from the hook approximately over the station point so that the plumb bob hangs about 2 cm above station point.

3) Bring the plumb bob exactly over the station point by moving each leg radially as well as circumferentially, and then press the legs firmly into the ground. By doing this the instrument is approximately leveled also.

4) If shifting head is provided the instrument, centering can be done rapidly. On hill side to ensure greater stability, place two legs of tripod down hill and the third leg uphill.

B)Leveling up of theodolite:

1) Turn the theodolite about its vertical axis until the plate level is parallel to any pair of leveling screws.

2) Bring the bubble to the centre of its run by turning both foot screws uniformly. By using thumb and forefingers, move the foot screws either towards each other or away from other.

3) Turn the instrument through 900 so that the bubble line will be at right angle to its previous position. Now, move only the third foot screw either in or out till the bubble is brought to the centre of its run.

4) Repeat the process until finally the plate bubble is exactly centered in both the positions.

5) Now rotate the theodolite about the vertical axis through 3600. The bubble will remain central provided it is in correct adjustment. The vertical axis is thus made truly vertical.

C) Focusing the eye piece :

The object of focusing the eye piece is to make the cross hairs on diaphragm distinct and clear. To do this, direct the telescope towards the sky or hold a sheet of white paper in front of the object glass, and move the eye piece circumferentially or in or out until the cross hairs are seen sharp and black.

D) Focusing of object glass : 

The object of focusing the glass is to bring the image of the object formed by the object glass exactly in the plane of cross hairs. If not done accurately, there will be an apparent movement of the image relatively to the cross hairs when the observer moves eye up and down. This effect is known as parallax. The parallax can be removed by the sharp focusing until the image appears sharp and clear. 

Answer 2

Temporary adjustment of theodolite:-

The following operations should be done as temporary adjustments before taking readings on any theodolite.

1. Setting of theodolite on tripod stand – The theodolite should be fixed by rotating its screw head on top of tripod stand. The legs of tripod stand should be fixed on ground very firmly to ensure safety of theodolite and easiness in taking observations.

2. Centring of theodolite over prefixed survey station – The centring of theodolite can be done by either dropping stone or suspending plumb bob from bottom of tri-batch plate.

Then it made to match over nail point of station peg by adjusting one of the legs. In some theodolite, optical plummet is provided for this centring.

3. Levelling of theodolite in horizontal plane – By keeping horizontal plate bubble tube (HPBT) parallel to any two foot screws, both are rotated inward or outward simultaneously to bring the bubble at center. Then by keeping HPBT perpendicular to original position, the third foot screw is rotated inward or outward to bring the bubble at center. These should be continued till in both positions, bubble of HPBT remains at center. Once bubble remains at center, levelling of theodolite is said to be completed.

4. Focussing of telescope – The focusing of telescope is done to remove parallax. First eyepiece screw is rotated to see clear image of cross-hairs. Then focussing screw is rotated to see clear image of object. Once both images (i.e. cross hairs and object) simultaneously focussing of telescope is said to be completed.