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Temporary adjustments of theodolites


Temporary adjustments are set of operations which are required to be done on an instrument (theodolite) in order to make it ready for taking observations. Temporary adjustments of theodolite include its setting up, centering, leveling up and elimination of parallax. Therefore these adjustments can be achieved in 4 steps:-

The setting operation includes fixing the theodolite with tripod along with approximate levelling and centering over the station mark. For setting up the instrument, the tripod is placed over the station with its legs widely spread so that the centre of the tripod head lies above the station point and its head approximately level (by eye estimation). The instrument is then fixed with the tripod by screwing through trivet. The height of the instrument should be such that observer can see through telescope conveniently. After this, a plumb bob is suspended from the bottom of the instrument and it should be such that plumb bob should point near to the station mark.

Centering implies that bringing vertical axis of theodolite immediately over station mark. To do this the following procedure is followed:-

It may be noted that due to radial movement of the legs, plumb bob gets shifted in the direction of the movement of the leg without seriously affecting the level of the instrument. On the other hand, when the legs are moved side ways or circumferentially, the plumb does not shift much but the level gets affected. Sometimes, the instrument and the tripod have to be moved bodily for centring. It must be noted that the centering and leveling of instrument is done recursively. Finally, exact centring is done by using the shifting head of the instrument. During this, first the screw-clamping ring of the shifting head is loosened and the upper plate of the shifting head is slid over the lower one until the plumb bob is exactly over the station mark. After the exact centring, the screw clamping ring gets tightened.

Leveling of an instrument is done to make the vertical axis of the instrument truly vertical. For accurate levelling the following steps are strictly followed:-

Otherwise, repeat the whole process starting from step1 to step5.

To obtain the clear reading, the image formed by the objective lens should fall in the plane of diaphragm and the focus of eye-piece should also be at the plane of diaphragm. This is being carried out by removing parallax by proper focusing of objective and eye-piece. Thus, focusing operation involves two steps:


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