The art of determining the relative heights of various points on or below the earth surface is understood as levelling.
Thus, levelling deals by means of measurements into the vertical plane.
Objects Of Levelling
The aim of levelling is to work out the relative heights of various objects on or below the earth surface and to work out the undulation on the surface of the earth.
Use Of Levelling
Levelling is completed for the following purposes:
1. To make a contour map for fixing sites for reservoirs, dams, barrages, etc. And to mend the alignment of roads, railways, irrigation canals, and so on.
2. To work out the altitudes of various details on a hill or to understand the reduced levels of various points on or below the surface of the world .
3. To arrange a longitudinal section and cross-sections of a project (roads, railways, irrigation canals, etc.) so as to work out the quantity of earth work.
4. to arrange a layout map for the supply of water, sanitary or drainage schemes.
Types Of Levelling Operations
1. Simple Levelling
When the difference of level between points is decided by setting the levelling instrument midway the points, the method is named simple levelling.
Let A and B are 2 points of difference of level is to be determined. the level is ready up at O, exactly midway between A and B.
Then accurate temporary adjustment, the readings of the staff on A and B are taken. The difference of those readings gives the difference of level between A and B (Fig. 5.10).
Differential levelling is adopted when
(i) the points are a good distance apart,
(ii) the difference of elevation between the points is large, and
(iii) there are obstacles between the points.
This method is additionally referred to as compound levelling or continuous levelling.
In this method, the level is ready up at several compatible positions and staff readings are taken in the least of those .
Consider Fig, 5. 1 1 . Suppose it’s required to understand the difference of level between A and B. the level is about up at points O1, O₂, O₃,etc.
Then transient adjustments, staff readinɡs are taken at every st-up. The points C1, C₂ and C₃ are referred to as chanɡe points.
Then the difference of level between A and B is observed. If the difference is positive, A is not up to B.
If it’s neɡative, A is more than B. Knowing the RL of A, that of B may be calculated.
3. Fly Levelling
While the differential levelling is finished consequently as to attach a bench-mark to the place to begin of the alignment of any project, it’s called fly levellinɡ.
And while the Fly levelling is additionally done to attach the BM to any intermediate point of the alignment for checking the accuracy of the work.
In such levellinɡ, only the backsight and foresight readings are taken at each found out of the level and no distances are measured along the direction of levelling (Fig. 5.12).
The level should be found out just midway between the BS and also the FS.
4. Longitudinal or Profile Levelling
The operation of taking levels along the centre line of any alignment (road, railway, etc.) at reɡular intervals is known as longitudinal levelling.
And hence in this activities, the backsight, intermediate sight and foresight readings are taken at regular intervals’ at every set-up of the instrument.
The chanɡes of the points are entered in the level book. This operation is taken on hand in apropos to determine the undulations of the ground surface along the profile line (Fig. 5.13).
5. Cross-sectional Levelling
The operation of taking levels transverse to the direction of longitudinal levelling, is understood as cross-sectional levelling.
The cross sections are taken at regular intervals (such as 20 m, 40 m, 50 m, etc.) along the alignment.
Cross-sectional levelling is completed so as to understand the character of the earth surface across the centre line of any alignment (Fig. 5.14).
6. Check Levelling
The fly levelling done at the end of a day’s work to attach the finishing point with the start line on it particular day is understood as check levelling.
It is Undertaken so as to examine the accuracy of the day’s work (Fig. 5.15).
Sources Of Error In Levelling
The following are the various sources of error or mistake in levelling:
1. Instrumental Errors
a) The permanent adjustment of the instrument might not be perfect. that’s the line of collimation might not be parallel to the axis of the bubble tube.
(b) The intestinal arrangement of the focussing tube isn’t perfect.
(c) The graduation of the levelling staff might not be perfect.
2. Personal Errors
(a) The instrument might not be levelled perfectly.
(b) The focussing of the eyepiece and object glass might not be perfect and parallax might not be eliminated entirely.
(c) The position of the staff could also be displaced at the change point at the of taking FS and BS readings.
(d) The staff may take place inverted while viewed through the telescope. By mistake, the staff readings is also taken upwards rather than downwards.
(e) The reading of the stadia hair instead of the central collimation hair could also be taken by mistake.
(f) A wrong entry could also be made within the level book.
(ɡ) The staff might not be properly and fully extended.
3. Errors because of Natural Causes
(a) When the space of sight is long, the curvature of earth may affect the staff reading.
(b) The outcome of refraction may cause a false staff reading to be taken.
(c) The effect of high winds and a shining sun may end in a wronɡ staff reading.
Permissible Error In Levelling
The precision of levelling is ascertained consistent with the error of closure permissible limit of closing error depends upon the character of labor that the levelling is to be done.
Permissible closing error is expressed as
E = closing error in metres,
C=the constant, and
D=distance in kilometres.
The following are the permissible errors for various forms of levelling:
1. Rough levelling—E = ± 0.100 √D
2. Ordinary levelling —E = ± 0.025√D
3. Accurate levelling—E = ± 0.012√D
4. Precise levelling —E = ± 0.006√D