Permanent Adjustments of A Level
The permanent adjustments of different level are made to establish the fixed relationships
between its fundamental lines. It indicates the rectification of instrumental errors
In a dumpy level, there are only two adjustments as the telescope is rigidly fixed to
the spindle.
1. The axis of the bubble tube should be perpendicular to the vertical axis
2. The line of collimation should be parallel to the axis of the bubble tube.
First Adjustment: To make the axis of the bubble tube perpendicular to the vertical axis. Object: The object of this adjustment is to ensure that if the instrument is once levelled up, the bubble remains in the centre of its run for all positions of the telescope. Necessity: The adjustment is made only for the convenience of taking readings quickly. Since it is necessary that the bubble should be central while taking any reading, much time is wasted if this adjustment is not made as in that case the bubble has to be brought in centre every time for each pointing of telescope. Test: (i) Set-up the level on firm ground and level it carefully by tripod-legs and foot-screws. The bubble will now be central in two positions at right angles to each other, one being parallel to a pair of foot-screw and the other over the third foot-screw. (ii) Bring the telescope over a pair of foot-screws or over the third foot-screw and turn it through 180 in the horizontal plane. If the bubble still remains central, the adjustment is correct.
Adjustment: (i) If the bubble does not remain in the centre, note down the deviations of the bubble from the centre, say it is ‘2n’ division over the bubble half way back i.e., ‘n’ divisions by raising or lowering end of the bubble tube by means of capstan headed must and the remaining half with the pair of foot-screws beneath the telescope at its present position. (ii) Turn the telescope through 90° so that it lies over the single foot- screw below the telescope or parallel to a pair of this screw or pair of foot -screws and bring the bubble in the centre of its run by means of this screw pair of foot-screws. (iii) Rotate the telescope and see if the bubble remains central for all positions of the telescope. If not repeat the whole process until the adjustment is correct.
Second Adjustment: To make the Line of collimation parallel to the axis of the bubble tube Object: The object of this adjustment is to set the line of collimation parallel to the bubble axis so that when the bubble is centered, the line of collimation should become exactly horizontal and not remain inclined as otherwise it would be. Necessity: The whole function of a level is to furnish a horizontal line of collimation, which is possible only if the above condition is satisfied. Test and Adjustments: The collimation error may be tested by any of the following three methods and then the necessary adjustments are made (concentrate on Two-Peg Method)
Two-Peg Method.: Test: (i) Drive two pegs A and B at a distance of (D) metres say 60 to 100 metres on a fairly level ground. Drive another peg at O exactly midway between A and B (ii) Set up and level the instrument at O and take the staff readings on A and B. The bubble must be in the centre while the readings are being taken. Let the staff readings on A and B, be a and b respectively. (iii) Shift the level and set it up a point O1 , d metres away from A (or B) and along the same line BA (Fig. 7.37). levels the instrument accurately and take staff readings on A and B with the bubble central. Let the readings be a1 and b1 respectively. (The level may also be set up at a point between A and B, d metres away from A or B)
(iv) Find the difference between the staff readings a and b, and that between the staff readings a1 and b1 . The difference of staff readings a and b gives the true difference in elevation between A and B as the instrument was set up exactly midway between A and B and that the back and for sight distances were exactly difference, whereas the difference between a1 and b1 gives the apparent difference. If the two differences are equal, the line of collimation is in adjustment, otherwise it is inclined and needs adjustment. Adjustment: (i) Find out whether the difference is a rise or a fall from the peg A to B. If a is greater than b, the peg A is lower than peg B and the ground is rising from A to B. If b is greater than a, the ground is falling from A to B. (ii) Find out the reading on the far peg B at the same level are of a1 by adding the true difference to a1 if it is a fall, or by subtracting the true difference from a1 if it is a rise. Let the reading be b2
(iv) The corrections to be applied for readings on the pegs A and B may be found out as under:
These corrections are additive if the of collimation is inclined downward and subtractive if the same is inclined upwards.
