what is the fundamental line of the theodolite

 Transit Theodolite


1. The verticle axis.

2.The axis of the plate level. (The plate level)

3.The line of collimation.

4.The horizontal axis (Turnion axis)

5. The axis of the altitude level. (The bubble line of altitude level)


                                            Why Necessary Permanent Adjustment

The improper and excessive use of the theodolite, the relationship between fundamental lines may be disturbed. This incorrect relationship affects on lead reading.

                                             Condition of Adjustment

A. The axes of the plate levels must be perpendicular to the vertical axis.

B. The line of collimation must be at the right angles to the horizontal axis.

C. The horizontal axis must be perpendicular to the vertical axis.

D. The axis of the telescope level or the altitude level must be parallel to the line of collimation.

E. If the transit has a fixed vertical vernier, The vertical circle vernier must be zero when the telescope level is centered. When the line of collimation is horizontal.

F. If the transit is provided with a string level must be parallel to the horizontal axis.

                                           To make the diaphragm truly erect

procedure:-Leveled the instrument carefully. sight a distant well-defined point such as the top of the spire, and with both motions clamped, rotate the telescope in azimuth by means of one of the tangent screw. If the horizontal cross-hair remains in contact with the point, the adjustment is correct. Alternatively, move the telescope through a small verticle angle. If the point travels continuously on the verticle hair, the adjustment is correct. If not, loosen the diaphragm screws and rotate the diaphragm- ring. Repeat the test and adjustment until perfect. Then carefully tighten the screws.

The permanent adjustment of the theodolite consists of the following.

a. Adjustment of the plate levels.

b. Adjustment of the line of collimation.

c. Adjustment of the horizontal axis.

d. Adjustment of the level tube of the telescope.

e. Adjustment of the verticle index frame.

For making the adjustment, the instrument should be set up at a fairly level place where sights of about 100m can be taken in either direction in the same straight line.


A. The axes of the plate levels must be perpendicular to the vertical axis.:-

Set the instrument on the firm ground and firmly fixed on the ground. Make leveling of plate level bubble by the use of a pair of foot screws. Turn the plate level at 90d about the verticle axis and make bubble central by the turn of third foot screw. Now the process is a repeat until the make the bubble center in these positions. Turn the plate level through 180d about verticle axis, if the bubble position is to remain central then the axis of the bubble is perpendicular to the vertical axis. If not the adjustment is required. Now note the no.of deviation of the bubble. Adjust half total no. of deviation by the use of capstan headed nut provided just below the bubble tube. The remaining half concerned foot screw. Repeat the process until making the bubble is central.

B. The line of collimation must be at the right angles to the horizontal axis.

To make the line of collimation coincide with the optical axis of the telescope. The first adjustment is in horizontal hair and the other is vertical hair.


Three pegs T, A, and B are driven in the ground. Set the instrument at A and make a temporary adjustment. Take the staff reading on A and B, Let staff reading on A is Aa and B is Bb1. Transiting the theodolite and take the staff reading A and B. Let the staff reading is on B is Bb2. If the second reading is matched with the first staff reading then horizontal hair is in adjustment, If not then the adjustment required in horizontal hair. Calculate the mean value of the two staff readings. Let the mean value is Bb. Turn the vertical diaphragm screw, The horizontal hair brought to the reading Bb.


Set the theodolite at T to make the temporary adjustment. The ranging fixed on A and looking by the use of the telescope, Clamped the lower and upper clamp. Fixed the ranging rod at B by using the transiting telescope. The upper clamp is loosed and by turning the vernier plate the ranging rod at A is again bisected. If the ranging rod at B is seen bisected after transiting the telescope then the vertical hair is perfect.if not BB1 is a total error. Shift the vertical hair by turning the horizontal diaphragm up to bisect the ranging rod at B1.

C. The horizontal axis must be perpendicular to the vertical axis

Set the theodolite at T from a distance pole P, To make the temporary adjustment. A is a point that marks on the pole P. The telescope bisect point A, clamp the upper and lower screw, then bisect the points B on the pole. Loosen the upper clamp turn the telescope through 180d. Then bisect point A telescope lowered and bisect the point B. If not another point B' is marked on a ranging rod at R. R is fixed at the same level of B.A point is C is marked midway between BB' The point C is bisected and the upper clamp is tightened. The telescope is raised then A is not bisect this time. Adjust the turning axis raised or lower until the bisecting point A.

D. The axis of the telescope level or the altitude level must be parallel to the line of collimation

When the level or verticle angles are taken then lines of collimation must be parallel to the axis of the telescope. Drive two pegs A and B on the ground about 100 mt apart. Set the instrument at O which is midway between two pegs. Clamp the verticle circle and bring the telescope bubble exactly to the center of its run by means of the tangent screw of the verticle circle. Take the reading the staff A and B. Find the true difference of levels A and B.

Set the instrument at O1 which is 10 mt from peg A. Make the temporary adjustment, and take the staff reading on A and B. Find the difference of level of A and B. If the true difference of level agrees with first setting then the adjustment is correct, If not calculate the true difference of and adjust the bubble by the help of capstan-headed screws attaching the level tube to the telescope.

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