Levelling is the art of determining the relative heights of various points on the surface of the earth. It is the branch of surveying that deals with the measurements in the vertical plane.
According to the method adopted, levelling may be classified broadly into two types namely,
- Direct levelling and
- Indirect levelling.
Let’s discuss each of the types of levelling in detail.
A) Direct levelling
In this method of levelling, the relative heights/elevations of points are found out by some direct observation.
Various types of levelling used in the direct method are
- Simple levelling
- Differential levelling
- Reciprocal levelling
- Longitudinal sectioning
- Cross sectioning
- Fly levelling
- Check levelling
Now, let’s discuss each of the types of levelling in detail.
1) Simple levelling
When the difference in level between any two points is determined by placing the levelling instrument midway between the points, then the process is called simple levelling.
By placing the instrument exactly at the midway,
- The error due to a false Line of sight (LOS) can be eliminated by balancing Fore Sight (F.S) and Back Sight (B.S) Reading.
- The error due to curvature and refraction also can be eliminated.
Let’s understand it through an example,
Let’s say A and B are two points whose difference in level is to be determined. The instrument is set up at O, exactly midway between A and B. Now take the staff reading at both points.
The difference in the staff reading gives the difference in level between A and B.
Also, if we know the RL of one station, then the RL of the other station can be easily calculated if we know the level difference between the two stations.
If you don’t understand the terminologies used here, then we highly recommend you to check out this article where the basics are clearly discussed.
2) Differential levelling
This type of levelling is also known as compound levelling (or) continuous levelling.
Differential levelling is adopted when
- The points whose difference in level is to be determined are at a great distance apart.
- The difference in elevations between the points is large.
In this method, the instrument level is set up at several suitable positions in between the major points, and staff readings are taken at all of these stations.
Let’s say, it is required to compute the level difference between A and B. Since, the two points are at a greater distance, the instrument level is set up at several in-between points like O1, O2, O3, and O4. The staff readings are taken at every setup.
By calculation, we can be able to found out the difference in level between A and B.
3) Reciprocal levelling
When it is not possible to set up the level instrument midway between two points due to some obstructions like river or lake, reciprocal levelling can be used.
Let’s consider A and B are the two points on the opposite side banks of the river.
First set up the levelling instrument near to A and take the staff readings on the staff held at A and B and name it as Sa and Sb.
Next set up the levelling instrument near to B and take the staff readings on the staff held at A and B and name it as Sa’ and Sb’.
Similar to simple levelling, in this method also, the following errors such as
- False Line of sight (LOS)
- Curvature error and
- Refraction error gets eliminated.
4) Longitudinal sectioning
The main objective of this type of levelling is to obtain a record of undulations of the ground surface along the centreline of proposed engineering projects like roads, railways, water line projects, etc.
The outline of the surface thus obtained from the observation is called the longitudinal section.
Longitudinal sectioning is also called as Profile levelling.
From such a section, the engineers can be able to study the relationship between the existing ground surface and the proposed formation of the new work along its direction.
Through this levelling, we can be able to know the
- Features of the original ground surface
- Formation levels of new proposed work
- Difference between the natural gradient and the proposed gradient
- Information about the depth of cutting and the height of filling of the earthwork.
5) Cross sectioning
Cross sections are run at right angles to the longitudinal sectioning. These lines are run on either side of the center line to provide the data regarding the lateral earth surface.
The length of the cross-sections depends upon the nature of the work. The cross sections are numbered consecutively from the beginning.
The longitudinal sections and the cross sections are worked together in the field and the data are recorded in the fieldwork.
6) Fly levelling
When the benchmark is too far away from the workstation and in order to connect the benchmark to the workstation, a particular type of levelling called fly levelling is adopted.
Fly levelling is used to connect the benchmark to the starting point (or) any intermediate point of the alignment.
Sometimes, fly levelling is also used for checking the accuracy of the work.
In fly levelling, only the back sight and fore sight readings are taken at every setup of the level instrument.
7) Check levelling
The fly levelling which is done at the end of the day’s work to connect the finishing point with the starting point of the alignment is known as check levelling.
It is undertaken to check the accuracy of the day’s work.
Let’s hope you understand every types of levelling used in the direct method. Now let’s discuss the indirect method of levelling in detail.
B) Indirect levelling
In this method of levelling, the relative elevations of various points are found out by some indirect relations and observations.
It may be carried out in the following three types of levelling,
- Barometric levelling
- Hypsometric levelling
- Trignometrical levelling
Let’s discuss each of the types of levelling in detail.
1) Barometric levelling
Barometric levelling is based on the principle that the atmospheric pressure varies inversely with the height i.e, when the height increases, atmospheric pressure decreases and vice versa.
When two points are at a greater distance and there is so much elevation difference, then we can easily measure the approximate elevation difference between the two points by the measurement of pressure at these points using a barometer.
This method gives approximate results and can be adopted in preliminary surveys.
2) Hypsometric levelling
Hypsometric levelling is adopted to find the relative elevations of various points by measuring the boiling point of the water at these points using a hypsometer.
Hypsometric levelling is based on the principle that the boiling point of the water decreases at higher altitudes.
This method also gives approximate results and can be adopted in preliminary surveys.
3) Trignometric levelling
In this method, the relative elevations of different points are calculated by measuring the vertical angle and the horizontal distance to the points.
FAQ
What is a level instrument?
A level instrument is used to establish a horizontal line of sight for observing staff readings. A level instrument consists of the following parts,
- A Telescope
- A Level tube
- A Levelling head
- A levelling staff
What are the types of levels?
There are different types of levels used in levelling. Some of the major ones are,
- Dumpy level
- Y level
- Tilting level
- Transit level
- Auto level
Click here to learn about each of them in detail.
Hope you understand everything you need to know about the Types of levelling in surveying. If you find this article helpful (or) If you have any queries, let us know in the comment section.
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