The theodolite is the most precise instrument designed for the measurement of horizontal and vertical angles and has applicability in surveying such as laying off horizontal angles ,locating point line,prolonging survey lines,establishing grades,determining difference in elevation ,setting out curves etc.
TYPES OF THEODOLITES
A transit theodolite is one is which the line of sight can be reversed by revolving the telescope through 180 in the vertical plane .It is mainly used theodolite.
NON TRANSIT THEODOLITE
The non-transit theodolite are either plain theodolite or Y-theodolites in which the telescope cannot be transited and non-transit theodolites have now become obsolete.
The vertical axis is the axis about which the instrument can be rotated in horizontal plane.This is the axis about which the lower and upper plates rotate.
The horizontal or trunnion axis is the axis about which the telescope and the vertical circle rotate in vertical plane.
The line of sight or line of collimation is the line passing through the intersection of the horizontal &vertical cross hairs & the optical center of the object glass & its continuation.
Transiting, plunging or reversing is the process of turning the telescope in the vertical plane through 180 about the trunion axis.Since, the line of sight is reversed in this operation ,it is also known as plunging or reversing.
Swinging the telescope is the process of the telescope in horizontal plane .If the telescope is rotated in clockwise direction ,it is known as right swing.I f the telescope is rotated in anticlockwise direction ,it is known as left swing.
Face left observation:-If the face of the vertical circle is to the left observer,the observation of the angle is known as face left observation.
Face right observation:-If the face of the vertical circle is to the right of the observer,the observation is known as face right observation.
TEMPORARY ADJUSTMENT OF THEODOLITE
Setting over the station:-The operation of setting up includes:-
CENTERING:-Centering of the instrument over the station mark is done by a plumbob or by optical plummet.By moving the leg radially,the plumbob is shifted in the direction of leg ¢ering is done by moving the leg circumferentally till the plumbob hangs within 1cm horizontally of the station mark.
APPROXIMATE LEVELLING:-This is done with reference to small circular bubble provided in tribarch.
LEVELLING UP:- Accurate leveling is done with the help of foot screws with respect to the plate levels.The purpose of the leveling is to make the vertical axis truly vertical.
Levelling up by three foot screws:-
1.Turn the upper plate until the longitudinal axis of the plate level is roughly parallel to a line joining any two of the leveling screws.
2.Hold these two leveling screws & turn them uniformly until the bubble is central.
3.Turn the upper plate through 90 until the axis of the passes over the position of the third leveling screw.
4.Turn this leveling screw until the bubble is central.
5.Return the upper plate through 90 to its original position & repeat step2 till the bubble is centered.
6.Turn back again through 90 and repeat step 4.
7.Repeat steps 2&4 till the bubble is central in the both the positions.
8.Now rotate the instrument through180.The bubble will remain in the centre.Thus, the vertical axis becomes truly vertical.If not it, needs permanent adjustment.
ELLIMINATION OF PARALLAX
Parallax is a condition ,when the image formed by the objective is not in the plane of the crosshairs.
Parallax can be eliminated in two steps :-
1.By focusing the eye-piece:-To focus the eye-piece for distinct vision of the cross-hairs,point the telescope towards the sky &move eye-piece in or out till the cross-hairs are seen sharp &distinct.
2.By focusing the objective:-The tekescope is now directed towards the object to be sighted &the focusing screw is turned till the image appears clear & sharp.The image so formed is in the plane of crosshairs.