10 Main Types of Levelling: Top 10 Types (With Diagram)| Surveying

The different types of Levelling are: 1. Differential Levelling 2. Check Levelling 3. Precise Levelling 4. Reciprocal Levelling 5. Longitudinal Levelling or Longitudinal Sectioning 6. Cross Levelling 7. Levelling for Giving Levels for Works 8. Barometric Levelling 9. Hypsometry 10. Trigonometrical Levelling.

Type # 1. Differential Levelling:

It is carried out with the object of determining the reduced levels of points some distance apart or to establish bench marks. The process has already been described in Continuous or compounds levelling and is also known as running flying levels from one point to the other.

In flying levelling, only the back and fore sights are necessary and the shortest convenient route between the points is selected. The length of a sight is kept as long as the power of the telescope and local obstacles permit.

Type # 2. Check Levelling:

It is conducted for the purpose of checking a series of levels, which have previously been fixed. At the end of each day’s work a line of levels starting from the point and returning to the starting point of that day is run with the object of checking the work done on that day.

Since the circuit is completed i.e., the levelling work ends at the starting point, therefore, for the work to be correct the difference between the sum of all the back sights and that of all therefore sights on that day should be zero.

Type # 3. Precise Levelling:

It is special method of levelling used for establishing bench marks with high precision at widely distant points, it is conducted by some govt., agency such as Great Trignometrical Survey of India department for establishing G.T.S. bench marks.

It requires the use of highly refined and modern instruments and greatest care in the field as described below:

Instruments:

A high grade level equipped with tilting screw, stadia wires and coincidence level etc. and an invar precision levelling staff are commonly used.

Precautions while Levelling:

(i) The adjustment of the level are carefully tested.

(ii) The parallax should be entirely eliminated by correct focussing.

(iii) The staff should be exactly vertical. It may be plumbed with the staff-level or plumb-bob.

(iv) The bubble should be exactly in the centre of its run at the time of taking readings.

(v) Lengths of sights are limiting to about 100 m.

(vi) The back sight and fore sight distances should be exactly equal. Stadia readings may be taken for this purpose.

(vii) Ground for level and staff should be stable. To avoid error due to settlement of tripod and staff, the back sights and the following fore sights should be taken in quick successions and the order of taking readings is interchanged at alternate set up i.e. at first setting, the back sight is observed first and then the fore sight while at the 2nd setting, the foresight is taken first and then the back sight and so on.

(viii) Levelling work should be suspended in rainy and windy days and also at noon in hot summer days. If work is necessarily to be done under such conditions, level should be protected from the sun or wind by a screen or umbrella.

(ix) Check levelling should be performed by a different surveyor on different days with different change points. And if the closing error exceeds the permissible value, the work should be repeated.

Type # 4. Reciprocal Levelling:

It is a method of levelling adopted to determine the difference of levels between two points when it is, not possible to set up the level midway between them as in the case of a river, a deep valley etc.

This method also eliminates:

(i) The error due to curvature and refraction, and

(ii) The error due to line of collimation not being parallel to the bubble line which may otherwise occur due to non —equality of back and fore sight distances.

The operation involves two sets of observations giving two incorrect differences of levels, the mean of which is the true difference. Let A and B be two points [Fig- 7.3 (a) and (b)] on opposite banks of a wide river.

Then to find the true difference of level between A and B, proceed as follows:

(i) Set up and level the instrument very near to A or over A [fig. 7.31. (a)].

(ii) Read the staff held at A and B and let the staff readings be (a₁) and (b₁) respectively.

(iii) Transfer the instrument to B and set it up and level very near to B or over B [fig. 7.31 (b)].

(iv) Observe the readings on the staff held at A and B and denote them as (a₂) and (b₂) respectively.

Note:

If the staff is near to the level, then the reading on it should be taken through the object-glass or the height of the centre of the eye-piece above that point should be measured directly with the staff and be considered as staff reading on that point.

Since the combined effect of collimation error, curvature and refraction in the readings is proportional to the distance of the staff from the instrument, the error in reading in both the cases are equal as the lengths of sight in each case is same.

Let in each case the reading at B be greater than that at A i.e., b₁ > a₁> b₂ > a₂.

and let, d₁ = incorrect difference of level of A and B., obtained from the observed readings while the instrument at A = b₁ —a₁.

d₂ = incorrect difference of level of A and B, obtained from the readings while the instrument at B = b₂ – a₂.

d = the true difference of level between A and B.

e = the total error due to imperfect adjustment of collimation, curvature and refraction.

Then the correct reading on B in the first case = b₁ —e

and that on A in the second case = a₂ — e

The reading on A in the first case (a₁) and that on B in the second case (b₂) are correct since the length of sight in each case is nil.

In the first case, the true difference, d = correct reading at B —correct reading at A (b₁ —e) a₁

Similarly, in the second case, the true difference, d = correct reading at B —correct reading at A:

i.e., the true difference of level between two points equals the mean of the two incorrect differences of level obtained in the two settings of the instruments.

Example :

In levelling between two points A and B on opposite banks of a river, the level was set up near A and the staff readings on A and B were 2.156 and 3.568 respectively. The level was then moved and set up near B, and the respective staff readings on A and B were 1.968 and 3.262. Find the true difference of level of A and B.

Solution:

Incorrect difference of level in the first case = 3.56- 2.156= 1.412

Incorrect difference of level in the second case = 3.262- 1.968= 1.294

True difference of level of A and B = mean of the two incorrect differences of level

=1.353m (fall from A to B) (Ans).

 

Type # 5. Longitudinal or Levelling:

The object of this levelling operation is to obtain a record of the undulations of the ground surface along the centre line of a proposed engineering project such as a road, railway or canal etc. The outline of the surface thus obtained is called the longitudinal section or profile so why this levelling is also known as profile levelling. From such a section, an engineer is in a position to study the relationship between the existing ground surface and the proposed formation of the new work in the direction of its length.

The operation involves observing the elevations of a number of points, along the centre line and also their distances along it. The line of section may be a single straight line running in the centre or may consist of a series of straight lines changing direction or connected by curves.

The levels are taken at uniform intervals of distance along the centre line depending upon the requirements of the work and nature of the ground. Besides these points the staff readings are taken at the fairly significant points where outline of the ground changes appreciably so that the profile may be obtained as natural as possible.

Running the Longitudinal Section:

The line of section is set out on the ground and marked with pegs driven at equal intervals usually 20 or 30 metres before starting the levelling operation. The levelling operations always commence at a bench mark and end on a bench mark.

If the permanent bench mark is not available near the line of section, a flying level is run from any permanent bench mark in the vicinity to establish a bench mark near the line of section. The level is set up in such a position as to command as many points on the section as possible.

Staff readings are the taken on pegs fixed already at desired regular intervals and also at significant points of change of slope. These readings are entered in the appropriate columns of the level-book against the respective changing along the line which are recorded in the distance column.

When it is necessary to shift the instrument on account of the length of sight being beyond the power of the telescope (usually 100 m) or the further points not being possible to be observed owing to the irregularities of the ground, a suitable change point is selected on firm ground or a well -defined permanent object and a fore sight is taken.

The change point may or may not lie on the line of section. The level is then shifted and set up in a new commanding position of chaining and observing the intermediate readings is obtained at the last point on the section. If the permanent bench mark exists near the end point of the section line, the work may be closed on it by running flying levels from the last station.

While running a section, it should be kept in view that position of the features such as a nallah, canal, river, road, railway, foot-path, cart-track etc., crossed by the section line are completely located and the arithmetical check is applied to the levelling work at the end.

Checking the Levels:

If the levelling starts from a permanent bench mark and ends on a permanent bench mark the difference between the sums of the back and fore sights must agree with the difference of levels between the first and the final bench mark within permissible limits of the closing error.

And if the levelling work is not closed on the permanent bench mark, the only way to check it is to take flying levels back to the original bench mark and finding the closing error. If the closing error exceeds the permissible value, the work must be repeated.

Plotting the L-Section:

In plotting the longitudinal section, a horizontal line is drawn as datum line and changes of the staff points are marked along this line to a convenient scale. At these plotted points, perpendiculars are erected and on each of these lines, the respective levels are set off. The plotted points are then joined by straight lines to obtain the outlines of the ground surface.

The horizontal scale used in plotting the distances of the points is the same as that of the plan but the vertical scale used in plotting the levels is always different and larger than the horizontal one so as to mark the inequalities of the ground more apparent. The horizontal scales commonly used are 10 m and 25 m to one cm and the vertical scale 1 m or 2 m to 1 cm.

The elevation of the datum line should be so assumed for each separate sheet that the length of the ordinates remain 15 cm. The assumed R.L. of the datum should be written on the datum line and the changes and the R. Ls. of the points are written against the perpendiculars. The datum and the ground lines are drawn in black and the perpendiculars in thin blue lines.

Working Profile:

When the design of an engineering project is made, a working profile is prepared for the use of engineer at site. It shows the features of the original ground surface, the formation levels of new work, the proposed gradient, the depths of cutting and heights of filling and any other information which may be useful during construction of the work.

The new work is represented by thick red line and the original ground by a comparatively thin burnt-sienna line. The natural surface levels are written in black or burnt-sienna and the formation levels in red. The gradients of new work are shown prominently and limits of each clearly shown by arrows. The depths of cutting are written in red and the heights of filling in blue. A road profile worked out from the assumed data (Table 7.13) is shown in fig. 7.33.

Type # 6. Cross Levelling:

The purpose of cross-sectioning is to determine the necessary information regarding the levels of the ground on either side of the longitudinal section for computing the quantities of earth work etc. for designing the engineering projects. Cross-sections are run at right angles to the centre line at 20 or 30 m intervals along the centre line.

The length of the cross-section depends upon the nature of the proposed work. In the case of an ordinary road the length may be 30 to 60 m on either side of the centre line and in the case of a railway, it may vary from 200 to 300 m or even more on either side of the centre line.

Taking Cross-Section:

Cross-sections are serially numbered from the start of the centre line and taken simultaneously with the longitudinal section. They are set out at right angles to the centre line with the chain or tape, the cross-staff, the optical square or the theodolite etc.

The staff is held at each 10m points and other points of sudden change of slope on the cross-section. Readings are then taken from the instrument stations used for the longitudinal section and the distance of the staff points measured with the tape towards left and right of the centre station.

The measurement of the cross-section distances may be entered in the level-book, R or L, written after each to indicate whether the measurement is to the right or
left of the centre line. If the longitudinal section measurements are to be entered in the distance column, the cross-distances should be written in the remarks column.

It is sometimes considered preferable to complete the longitudinal section first and then taking the cross-sections so as to ensure correctness of the profile work through necessary checks before taking cross-sectioning in hand. In such a case, the levelling operation for cross-sectioning starts either from the nearest bench mark of from each central peg, the level of which is previously determined during longitudinal sectioning.

In either case, the instrument is set up to command as many cross-sections or points there on as possible and the staff reading taken and recorded as usual. To avoid confusion, the booking of each cross-section should be entered on a separate page of the level –book.

Plotting the Cross-Sections:

The cross-sections are plotted in the same manner as the longitudinal section except that both the horizontal and vertical measurements are plotted to the same scale as is commonly used for the vertical dimensions of the profile i.e., 1 m or 2m to 1 cm.

The R.L. of the datum line may be different for different sections to keep the ordinates fairly short. A cross section for the assumed data (Table 7.14) at change 60 of the Longitudinal section of the previous problem is shown in Fig. 7.34.

Type # 7. Levelling for Giving Levels for Works:

In the execution of works, it often becomes necessary to give levels or marks up to which a work is to be constructed. This is done by driving a peg or making a mark either at a desired level or at a known distance above or below it. In the first case, knowing the R.L. of the desired point and the height of the instrument (H.I.), the staff reading required at that point is first calculated by subtracting the R.L. of the desired point from the height of the instrument (H.I.).

The staff is then lowered or raised until the desired reading is obtained and the point is marked suitably. In the second case, the staff reading of an arbitrary point is first ascertained as explained above and difference between that point and the actual point of construction is determined.

To avoid confusion, this difference may be an exact number of metres. Levels given by this method are transferred to the actual points of construction afterwards when required. This may be done by using sight rails and bonding rods or a straight edge and a spirit level.

Type # 8. Barometric Levelling:

It is a process of levelling based up on the fact that the atmospheric pressure varies inversely with the height. In this method, a barometer is used to measure the atmospheric pressure and thus the elevation of points. It is not an accurate method and so is chiefly used for rough type levelling works such as during reconnaissance.

Type # 9. Hypsometery:

This method of levelling is based upon the principle that the boiling point of water changes with the height. Hypsometer is used to find the boiling points of water and thus the height of the points. It is also an approximate method.

Type # 10. Trigonometrical Levelling:

It is the method of levelling in which the heights of points are calculated from the horizontal distances and vertical angles measured in the field.