The methods of setting up the theodolite and observing angles will now be dealt with. It should be emphasized, however, that these instructions are no substitute for practical experience.

 

Setting up using a plumb-bob:

Figure shows a theodolite set up with the plumb-bob suspended over the survey station. The procedure is as follows:

(1) Extend the tripod legs to the height required to provide comfortable viewing through the theodolite. It is important to leave at least 100 mm of leg extension to facilitate positioning of the plumb-bob.

(2) Attach the plumb-bob to the tripod head, so that it is hanging freely from the centre of the head.

(3) Stand the tripod approximately over the survey station, keeping the head reasonably horizontal.

(4) If the tripod has them, tighten the wing units at the top of the tripod legs and move the whole tripod until the plumb-bob is over the station.

(5) Now tread the tripod feet firmly into the ground.

(6) Unclamp a tripod leg and slide it in or out until the plumb-bob is exactly over the station. If this cannot be achieved in one movement, then use the slide extension to bring the plumb-bob in line with the survey point and another tripod leg. Using this latter leg, slide in or out to bring the plumb-bob onto the survey point.

(7) Remove the theodolite from its case and holding it by its standard, attach it to the tripod head.

8) The instrument axis is now set truly vertical using the plate bubble as follows:

(a) Set the plate bubble parallel to two footscrews A and B as shown in (Figure 5.28(a)) and centre it by equal amounts of simultaneous contra-rotation of both screws. (The bubble follows the direction of the left thumb.)

(b) Rotate alidade through 90◦ (Figure 5.28(b)) and centre the bubble using footscrew C only.

(c) Repeat :

(a) and (b) until bubble remains central in both positions. If there is no bubble error this procedure will suffice. If there is slight bubble error present, proceed as follows.

(d) From the initial position at B (Figure 5.28(a)), rotate the alidade through 180◦; if the bubble moves off centre bring if half-way back using the footscrews A and B.

(e) Rotate through a further 90◦, placing the bubble 180◦ different to its position in Figures 5.28(b). If the bubble moves off centre, bring it half-way back with footscrew C only.

(f) Although the bubble is off centre, the instrument axis will be truly vertical and will remain so as long as the bubble remains the same amount off centre .

(g) Test that the instrument has been correctly levelled by turning the instrument to any arbitrary direction. If the instrument is correctly levelled the bubble will remain in the same position within its vial no matter where the instrument is pointed.

 

(9) Check the plumb-bob; if it is off the survey point, slacken off the whole theodolite and shift it laterally across the tripod head, taking care not to allow it to rotate, until the plumb-bob is exactly over the survey point.

 

(10) Repeat (8) and (9) until the instrument is centred and levelled.

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