The permanent adjustments of different level are made to establish the fixed relationships between its fundamental lines: 1. Permanent Adjustments of a Dumpy Level 2. Permanent Adjustments of a Cooke’s Reversible Level 3. Permanent Adjustments of a Cushing’s Level 4. Permanent Adjustments of the Y-Level 5. Permanent Adjustments of a Tilting Level.
1. Permanent Adjustments of a Dumpy Level:
In a dumess level, there are only two adjustments as he 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.
ADVERTISEMENTS:
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.
ADVERTISEMENTS:
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 the 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.
ADVERTISEMENTS:
(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.
ADVERTISEMENTS:
(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:
ADVERTISEMENTS:
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:
First 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 (Fig. 7.36)
(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 (Fig. 7.38).
(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.
(iii) If b1 is greater than b2, the line of collimation is Inclined upwards and if b1 is smaller than b2, the line of collimation is inclined downwards. b1 – b2 (difference of b1 and b2) is the collimation error in the distance “D”.
∴ the collimation error for unit distance:
(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.
Remember:
The difference between the corrected readings will be equal to true differences of level.
Alternative method for finding the correct staff readings is given as under:
(a) Let α be the angle of inclination of the of collimation to the horizontal and suppose that line of collimation is inclined upwards (Fig .7.37) and the peg B is lower than peg A
Then ef= d tan α, and gh = (D+d) tan α
Correct staff reading on peg A=AF-ef= d1-d tan α,
‘’ ‘’ ‘’ ‘’ ‘’ ‘’ ‘’ ‘’ B=Bh – gh
= b1-(D+d) tan α (The true differences in elevation between)
A and B= [{b1-D+d) tan α} – {α1-d tan α}]
This differences is equal to the differences (b-a):
... [Σ [{b1-D +d) tan α } – {α1-d tan α }] = b-a
(b) Find the value of tan from the above equation.
If it is positive, the assumption is correct and the line of collimation is inclined upwards; if negative it is inclined downwards.
(v) The adjustment is made by adjusting the diagram by the diagram-screws so that the correct staff readings are obtained when the staff is held on the pegs A and B. The adjustment is usually made by keeping the staff on the far peg and checked on the near peg.
To apply the correction, keep the staff on the far peg and see whether the correct staff reading is seen above or below the horizontal hair. If the correct reading is less than the observed one (b1), this will be seen above the horizontal hair when seen through the telescope and the cross-hair has to be raised from its true position. To do this, loosen the lower screw and tighten the upper one till the intersection coincides with the correct staff reading. But if the correct reading is greater than the observed one (b1), this will be seen below the horizontal hair, the cross-hair has to be lowered.
To do this, loosed the upper screw and tighten the lower one.
It may be noted that the cross-hair moves towards that screw which is being tightened.
Remember the rule of three Ls, Less Loose Lower i.e. if the reading seen through the telescope is to be lessened, then loosen the lower screw and tighten the upper one and vice versa.
(vi) Now place the staff on the near peg (A) and take the staff reading. If the adjustment is correct, this reading should agree with the calculated correct reading on the peg A, otherwise repeat the adjustment until perfect.
Second Method:
(i) The first two steps are the same as in the two- peg method.
(ii) Find the true difference in elevation (a – b) between A and B. If a is greater than b there is a rise from A to B, and if b is greater than a there is a fall from A to B.
(iii) Shift the instrument and set it up and level very near to A (or B). Observe the staff readings a and b on A and B respectively. The reading a (or b) on the near peg A1 (or B1) shall have to be taken by looking through the object-glass. The reading a1 is the correct staff reading on A as the instrument is set up just by the side of the peg A.
(iv) Find out the correct staff reading at the far peg B at the same level as of a1 by adding the true difference to if the ground is falling from A to B or by subtracting the true difference from a1 if the ground is rising from A to B.
... Correct staff reading on the peg B = a1 ± true difference, use plus sign, if it is fall from A to B and minus sign, if it is rise from to B.
(v) If the observed staff reading b1 on peg B is equal to the calculated correct reading, the line of collimation is in adjustment. If b1 is greater than the calculated staff reading, the line of collimation is inclined upwards and if b1 is less than the calculated staff reading, the line of collimation is inclined downwards.
Adjustment:
The adjustment on the far peg B is made in the same way as explained in the two-peg method.
Third Method:
In this method, the principle of reciprocal levelling is made use to determine the true difference of level between the pegs A and B. Having determined the true difference of level between A and B obtain the correct staff readings on A and B as in the second method. The adjustment is then made as described in the two- peg method.
2. Permanent Adjustments of a Cooke’s Reversible Level:
By slackening the stop-screw, the telescope of a Cooke’s reversible level can be rotated about its longitudinal axis in the sockets and can also be withdrawn from the socket s and replaced end for end.
There are three adjustments for this level viz:
1. The line of collimation should coincide with the axis of the telescope.
2. The line of collimation should be at right angles to the vertical axis.
3. The axis of the bubble tube should be perpendicular to the vertical axis.
First Adjustment:
To make the line of collimation coincident with the axis of the telescope.
Necessity:
If this condition is not satisfied, the line of collimation will generate a cone when rotated about its longitudinal axis and the axis of the bubble tube will not remain parallel to it for all positions of the telescope, unless the telescope can be fixed in one position. Therefore this condition is the first necessity for the property of reversibility of the instrument.
Test:
(i) Set up the level at a distance of 30 to 60 m from a wall and level it. The instrument is so set up that the telescope lies over one foot- screw when directed towards the wall.
(ii) Direct the telescope towards the wall and focus it. Mark a point A on the wall coinciding with the intersection of the cross-hairs (Fig.7.46)
(iii) Slacken the stop-screw and withdraw it and rotate the telescope about its longitudinal axis through 180° so that the stop-screw-hole is brought to the top and the cross-hair is horizontal. See whether the point A is bisected again or not. If A is bisected, then the instrument is in adjustment.
(iv) Otherwise, mark another point B coinciding with the cross-hair, above or below the point A.
(v) Measure the distance AB accurately. The actual error is half the vertical distance AB. Mark a point C exactly midway between A and B.
Adjustment:
(i) Move the diagram up or down by means of the diagram screws until the mark C is bisected by the cross-hair. If the diagram is to be raised, loosen the lower screw and tighten the upper one and vice versa.
(ii) Repeat the test and adjustment until the adjustment is correct.
Alternatively, instead of making mark A, B and C on the wall, a staff may be held at a distance of 30 to 60 m from the instrument, a reading coinciding with the cross-hair (say a1) is first taken. The telescope is then rotated through 180° and the second reading on the staff (say b1) is observed.
If the two readings are equal, the instrument, is in adjustment, if not, the correct reading will be
. Then adjust the diagram to bisect this reading on the staff.
Second Adjustment:
To place the Line of Collimation a right angles to the vertical axis.
Necessity:
The main condition that the line of collimation should be parallel to the axis of the bubble tube is attained indirectly by making each of these, i.e. the line of collimation as well as the bubble axis perpendicular to the vertical axis.
Test:
(i) The first two steps are the same as in the first adjustment of this level.
(ii) Remove the stop’-screw and carefully withdraw the telescope from its socket. Turn the socket end for end and gently replace the telescope. See that the cross-hair is exactly horizontal.
(iii) Again sight the test mark A and see if it is bisected again not. If A is bisected, the adjustment is correct.
(iv) If not, mark another point B coinciding with the cross-hair above or below the point A.
(v) Measure AB and mark a point C exactly midway between the two.
Adjustment:
The adjustment is made by means of the base plate or limb-nuts:
(i) Raise or lower the socket by means of the base-plate limb-nuts until the point C is bisected by the cross-hair.
(ii) Turn the foot-screw beneath the telescope until the cross-hair again bisects the mark A. Now reverse the telescope end for end and see whether the mark A is still bisected by the cross-hair.
(iii) If not, repeat the test and adjustment till correct.
Alternatively, instead of marking points A, B and C on the way the staff readings a1 and b1 may be observed and the socket is adjusted such that the cross-hair bisects the reading 
Third Adjustment:
To make the axis of the bubble tube perpendicular to the vertical axis.
Test and Adjustment:
The test and adjustment are the same as in the first adjustment of the dumpy level. This adjustment was made only for convenience of taking quick readings in the dumpy level but in the case of this level, it is a necessity because the parallelism of the line of collimation and the bubble axis also depends on this adjustment.
3. Permanent Adjustments of a Cushing’s Level:
In Cushing’s level, the telescope can neither be rotated about its longitudinal axis nor it can be withdrawn from its socket and changed end for end directly, but both the above properties of the telescope of this level are attained indirectly by making the end collars exactly similar which can be interchanged to reverse the telescope end for end and also fixed after the desired rotation in its fittings.
The fundamental lines of this level and the relations between them are the same as those for Cooke’s reversible level. The methods of testing and adjusting are also exactly similar to those employed in the case of Cookes’ in the level.
First Adjustment:
To make the line of collimation coincident with the axis of the telescope.
Test and Adjustment:
This is made in the same way as the first adjustment of Cooke’s level except that in step (iii), rotate the eye-piece end (i.e. eye-piece and diagram) through 180° in its fittings instead of rotating the telescope about its axis through 180°.
Second Adjustment:
To place the Line of collimation at right angles to the vertical axis.
Test and Adjustment:
Similar to the second Adjustment of Cooke’s level except that interchange the eye-piece and objective ends instead of reversing the telescope end for end.
Third Adjustment:
To make the axis of the bubble tube perpendicular to the vertical axis.
Test and Adjustment:
Performed in the same manner as the first adjustment of dumpy level.
4. Permanent Adjustments of the Y-Level:
In the Y-level, the telescope can be revolved about its longitudinal axis. It can also be lifted from the Y-supports and changed end for end.
There are three permanent adjustments viz.:
1. The line of collimation should coincide with the axis of the telescope.
2. The line of collimation should be parallel to the axis of the bubble tube.
3. The axis of the bubble tube should be perpendicular to the vertical axis.
First Adjustment:
To make the line of collimation coincident with the axis the telescope. The instrument is manufactured such that the axis of the telescope collars coincide with the optical axis of the telescope and also it is parallel to the bearing surface of the collars.
Therefore the line of collimation should coincide with the axis of the telescope collars or parallel to bearing surface of the collars. This adjustment is essential before performing the second adjustment in which the bubble axis is made parallel to the bearing surface, but with this adjustment it becomes parallel to the line of collimation.
Test and Adjustment:
Similar to the first adjustment of the Cooke’s level except that for rotating the telescope through 180-, loosen and raise wye-clips and turn the telescope about the longitudinal axis.
Second Adjustment:
To make the Line of collimation parallel to the axis of the bubble tube. With this adjustment, the line of collimation remains horizontal when the bubble lies in the centre of its run. This is very important relationship as it constitutes the fundamental principle (to furnish horizontal line of sight) of spirit levelling.
This adjustment is made in two stages:
(1) The axis of the bubble tube is placed in the plane of the line of collimation,
(2) The axis of the bubble tube is made parallel to the line of collimation (or the bearing surface of the collars).
First Stage:
To place the axis of the bubble tube in the plane of the line of collimation.
Test:
(i) Set up the instrument and level it accurately.
(ii) Loosen and raise the wye-clips. Revolve the telescope about its longitudinal axis through a small angle, (say 10°). If the bubble still remains central, the adjustment is correct.
Adjustment:
If the bubble does not remain central, bring it entirely back to the central position by means of the azimuth screw. The bubble tube is thus adjusted laterally. It may be noted here that in this case the whole deviation of the bubble is the actual error since no reversal is made in the test. Repeat the test and adjustment till it is correct.
This adjustment is necessary only for those Y-levels in which the level tube is fitted with the azimuth screw.
Second Stage:
To make the axis of the bubble tube, parallel to the line of collimation (or the bearing surface of the collars.).
Test:
(i) Set up and level the instrument and bring the telescope over one foot-screw and clamp it. Bring the bubble to the centre of its run by means of this screw.
(ii) Loosen and raise the Y-clips. Lift the telescope gently, turn it end for end and replace it in wyes. If the bubble remains central, the adjustment is correct.
Adjustment:
(i) If the bubble is deviated, note its deviation from the centre. Let it be 2n divisions. Then the actual errors is n divisions.
(ii) Bring the bubble half way back towards centre i.e. n division by means of the level-tube nuts and the remaining half by the foot-screw beneath the telescope.
(iii) Repeat the test and adjustment till correct.
Note:
Since the reversal of the telescope is made about-the Ys, the line of collimation is made parallel to the line joining the bottom of the Ys. (i.e. parallel to the bearing surface of the collars). The line of collimation will be parallel to the axis of the bubble tube only if the end collars are similar and the first adjustment is made accurately. Therefore the above adjustment fails in cases where the diameters of the end collars are not exactly equal due to uneven wear of the bearing surfaces.
In such a case, the two-peg method as already explained in the adjustment of dumpy level may be employed. The method may be used in the same way except that the intersection of the crosshairs is brought to the calculated correct staff reading on the far peg by means of levelling screw beneath the telescope. The line of collimation is now horizontal but the bubble is displaced from its central position. Then bring the bubble to the centre by means of level tube nuts.
Third Adjustment:
To make the axis of the bubble tube perpendicular to the vertical Axis. The method is similar to that of the first adjustment of dumpy level except that the bubble is brought half way back by Y-nuts and the remaining half by foot-screw under the telescope.
5. Permanent Adjustments of a Tilting Level:
In the tilting level, the telescope along with the main bubble tube can be tilted by means of the tilting screw independent of the vertical axis. Therefore, there is only one adjustment viz, the axis of the bubble tube should be parallel to the line of collimation so that the line of collimation is horizontal when the bubble lies in the centre of its run.
There are two types of the tilting level:
(1) Reversible, and
(2) Non-Reversible.
In the reversible type, the telescope along with the main bubble tube can be rotated about its longitudinal axis through 180 whereas in non-reversible type, the telescope cannot be rotated as above.
Adjustment:
To make the axis of the bubble tube parallel to the line of collimation:
(1) Reversible Type:
Necessity:
Same as for second adjustment of the dumpy level.
Test:
(i) Set up the level such that the main level tube is on the left face of the instrument. Level it approximately by foot-screw. Hold the staff at a distance of about 100 m.
(ii) Bring the main bubble exactly to the centre of its run by using the tilting-screw and take the staff reading. Let the reading be a1.
(iii) Rotate the telescope about its longitudinal axis through 180° so that the main bubble tube is now on the right face of the instrument.
(iv) After making the bubble exactly central, again read the staff.
Let the reading be a2. If the two readings agree, the adjustment is correct.
Method of Adjustment:
(i) If the two readings disagree rotate the telescope to the original position so that the bubble tube is again on the left face of the instrument and then set the telescope to the mean of the above readings by turning the tilting screw. The line of collimation is now horizontal but the bubble becomes out of centre.
(ii) Bring the bubble exactly to the centre of its run by means of the screws attached with the bubble tube.
(iii) Repeat the procedure until correct.
(2) Non-Reversible Type:
Necessity:
Same as for the second adjustment of the dumpy level.
Test:
Same as the two-peg test for the second adjustment of the dumpy level.
Method of Adjustment:
(i) If the apparent and true differences of level are not equal, find the correct staff readings on the two pegs.
(ii) Hold the staff at the far peg B and set the telescope to the corrected staff reading by means of the tilting-screw. The bubble is divided from its central position.
(iii) Bring the bubble to the centre of its run by means of bubble adjusting screws.
(iv) Repeat the operation until correct.