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Keywords: Levelling, Direct levelling, Trigonometric levelling, Barometric levelling, Hypsometric levelling, Simple levelling, Differential levelling/Compound levelling, Check levelling, Profile levelling, Cross-section levelling, Precise levelling, Reciprocal levelling
Now we can move forward and discuss about the different methods of levelling. They are as follows:
i. Direct levelling
This is a most common and widely used method in levelling.This is also known as spirit levelling. Because a spirit level is mounted on the telescope of the levelling instrument which is used to make the line of sight horizontal. The vertical distances are measured with respect to this horizontal line of sight and are used to compute the difference in elevations of various points.
ii. Trigonometric levelling
It is also known as indirect levelling. Because the elevations are determined indirectly from the horizontal distances and vertical angle measured at the point. As trigonometric relations are used to determine the elevations, it is called as trigonometric levelling. This is generally used when direct levelling becomes difficult, such as at the elevations of inaccessible points like mountain peaks or top of towers etc.
iii. Barometric levelling
The elevations are determined indirectly from the changes in atmospheric pressure. The atmospheric pressure decreases with an increase in elevation. Examples are the aneroid barometer, which can be used for determining the changes in atmospheric pressure. Also called the altimeter, it is quite light, sturdy and convenient compared to the mercury barometer, but not as accurate. It is quick method of levelling with accuracy to the nearest 1 -2 meters.
iv. Hypsometric levelling
In this method the difference of elevations is obtained by noting down the temperature at which water starts boiling. This is because, the boiling point of water decreases with increase in altitude.
Classification of Direct levelling methods
Since, you have got fairly good knowledge about the different methods of levelling, we can now move on to discuss more about the types of direct levelling methods.
Direct levelling methods are classified as follows:
i. Simple levelling
This is the easiest type of direct levelling. Because it only need one set up of the levelling instruments.Thisis commonly adopted for determining the difference in elevations of 2 points visible from a single position of the instrument.
ii. Differential levelling/Compound levelling
This type of levelling needs more than one set up of the levelling instruments. This is used to find the difference of elevations of two points which are situated at a large distance apart or the difference in elevations of the two points is large. This is also referred as “taking fly levels”.
iii. Check levelling
This type of levelling is adopted for the purpose of checking a series of levels previously fixed. This is often done at the end of a day to check the accuracy of the work by returning to the starting point of the day.
iv. Profile levelling
This is used for determining the elevations of points at known distances which are apart, along a given line to get an accurate outline of the whole surface of the ground. This also called as longitudinal levelling or sectioning. Profile levelling is donealong the center line of the proposed route (road, canal, railway line, sewer line etc.) It is used for plotting the longitudinal section,which is useful for fixing the gradients and for determining the earthwork quantities.
v. Cross-section levelling
This type of levelling is done to determine the difference of elevations of the ground surface lying along the perpendicular to the center line of a proposed road,canal etc.
The cross-section levelling is required to determine the configuration across the alignment. Cross-section of a plot is helpful for determining the quantities of earthwork.
vi. Precise levelling
This is a very accurate method of differential leveling which is usually used whenever high precision is required. Also the instruments and methods used are of high precision too. Due to that this is a costly method.
vii. Reciprocal levelling
In this method of levelling in which the difference in elevations between two points is accurately determined by two sets of observations where it is not possible to set up the levelling instrument midway.