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Unit - 4

Theodolite Survey

Q1) What are the components of transit theodolite?

A1) A transit theodolite essentially consists of following:

The levelling head

The two spindles

The lower circular metal plate

Vernier plate or upper plate

The telescope

The level tube

The standards

The vertical circle

The vernier frame

The compass

Clip screw

Optical plumet

Eyepiece

Shifting head

Bubble tube

Lower tangent screw

Q2) What are the uses of theodolite?

A2)

Theodolite makes use of for plenty purposes, however particularly its miles used for measuring angles, scaling factors of constructional works. For example, to decide motorway factors, massive buildings’ escalating edges theodolites are used.

Depending at the activity nature and the accuracy required, theodolite produces extra curved readings, the usage of paradoxical faces and swings or special positions for ideal measuring survey.

Followings are the important makes use of of theodolite:

Measuring horizontal and vertical angles

Locating factors on a line

Finding the distinction withinside the level

Prolonging survey lines

Ranging curves

Setting out grades

Tachometric surveying

Q3) What Temporary adjustments are done for theodolite?

A3)

The following are the temporary adjustments:

Setting up the theodolite over a station

Levelling up

Focusing of eyepiece and object glass for elimination of parallax

(a) Setting up the theodolite over a station:

Setting up of the theodolite includes following process:

(a) Centering the instrument over a station point

(b) Levelling the instrument approximately by the tripod legs

Centering the instrument over a station point:

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

Plumb bob is suspended from hook approximately over the station mark.

By moving each leg radially as well as circumferentialy, bring the plumb bob exactly over the station point, then press the legs firmly into the ground.

By doing this the instrument is approximately levelled. If the instrument has a shifting head, centering can be done rapidly.

On hill side to ensure greater stability, place two legs of tripod down hill and third leg uphill.

(b) Levelling the instrument approximately by the tripod legs:

After the approximate levelling by leg adjustments, the accurate levelling is properly done with the help of foot screws.

The accurate levelling is essential so as to make the vertical axis truly vertical.

Accurate levelling with the help of three-foot screws includes the following steps:

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

Move foot screws either both outward or both inwards simultaneously till the bubble comes in the center of its run.

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Fig.: Levelling up with three-foot screws ‘PQR’

3. Rotate the telescope in 90° and then bring it over the third foot screw and bubble is brought in the center of its run by operating the third foot screw either clockwise or anti clockwise.

4. Again, bring the telescope back to its original position as it was in step 2 and check the centering of the bubble. Again, move the pair of foot screws either both outwards or both inwards simultaneously till the comes in the center of its run.

5. Again, turn the telescope through 90° over the third foot screw and check whether the bubble remains in center or not. If not, repeat these operations till the bubble remains in center for both positions of the telescope.

6. Now turn the telescope in any position through 360° and observe the bubble. If the bubble remains in center for all positions of the telescope, then telescope is now in accurate levelling and ready for further work of levelling.

(c)Focusing of eyepiece and object glass to eliminate the parallex:

(i) Focusing of eyepiece:

Focusing of eyepiece enables to make the cross hair clear and distinct.

This is achieved by holding a sheet of white paper in front of the object glass and the eyepiece is rotated in clockwise or anticlockwise direction till the cross hairs are clear, sharp and distinct.

(ii) Focusing the object glass

The object of focusing the object glass is to bring the image of the object formed by the object glass in the plane of the cross hairs.

To eliminate parallax, direct the telescope towards the object and tum the focusing screw add the image appears clear and sharp.

Q4) How can we measure horizontal angle?

A4)

Setup the theodolite over the station Q and level it carefully.

Set the vernier A to the zero of the horizontal circle, the horizontal circle is usually graduated from 0° to 360°.

$H:\fig1\IMG_20210604_185338.jpg$

Fig.: Measuring Horizontal angle

3. In order to set vernier A to the zero, loose the upper plate until the arrow of the vernier A approximately coincides with the zero of the horizontal circle. Clamp both plates together and by operating the slow-motion screw bring the arrow into exact coincidence with the zero of the circle. Both plates are now clamped.

4. Loose the lower clamp. Turn the instrument and then bisect the ranging rod at P. Tighten the clamp and bisect P exactly by using the lower slow-motion screw. The point of intersection of cross hairs should be brought into exact coincidence with the station point rod.

5. Check the reading of the vernier it should be same as above i.e., zero.

6. Now unclamp the upper plate and turn the telescope in clockwise direction until it bisects the ranging rod at R. Tight the upper clamp and by using the upper tangent screw, bisect the ranging rod at R exactly.

7. Read both verniers. The reading on vernier A gives the value of the angle PQR, whereas reading on vernier 'B' must be deducted from 180°, in order to get value of the angle PQR.

8. The mean of the two verniers reading will give required value of PQR.

9. Change the face of the instrument and repeat the whole process. The mean of the two vernier reading will give required value of PQR.

10. On all important work take both face observations to eliminate the errors due to imperfect adjustment of the instrument.

11. The mean of both face reading gives exact value of angle PQR which is free from all instrumental errors.

Q5) How can we measure vertical angle?

A6)

To measure the vertical angle of an object P at a section O.

Set up the instrument over station O and level it carefully with respect to altitude bubble.

By means of vertical circle clamp and tangent screw, set zero of the vertical vernier exactly to the zero of the vertical circle.

Fig.: Measuring vertical angle

4. Bring the bubble of the altitude level to the center of its run by means of the foot screws.

5. The line of collimation is thus made horizontal, while the vernier reads zero.

6. Loosen the vertical circle clamp and direct the telescope in vertical plane towards the object 'P' and bisect exactly using the vertical circle tangent screw.

7. Read both the verniers C and D. The mean of the two readings gives the values of the required angle for this face.

8. Change the face of the instrument and repeat the process. The mean of two vernier reading gives the values of the required for this face.

9. The average of two values obtained from left observation and face right observation gives exact values of the required angle.

10. Errors eliminated: The average of two values thus obtained gives required angle which is from instrumental error.

Q6) What is theodolite traversing?

A6)

The technique of repetition used to degree traverse angles to a finer diploma of accuracy than that workable with the least be counted number of the vernier geared up at the theodolite.

In this technique, an attitude degree there or 4 instances through maintaining the vernier clamp while sighting on the lower back station.

While swinging from forwarding station to lower back station, the top plate is set free and made unfastened to rotate.

Thus, an attitude analyzing routinely provides as commonly because the wide variety of repetitions. The distinction among the primary and the ultimate analyzing 5 the incorporated traverse attitude and the common traverse attitude then acquired through dividing the incorporated attitude through the wide variety of repetitions.

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