Test: Methods of Levelling - Civil Engineering (CE) MCQ

The 2 methods are the height of instrument method and rise and fall method. These are used to evaluate heights of various points.

The following corrections are to be applied while determining the difference in elevation between two points when the observations are from only one point:

• Axis signal correction
  • In order to observe the points from the theodolite station, signals of appropriate heights are erected at the points to be observed. The signal may or may not be the same height as that of the instrument.
  • If the height of the signal is not the same as that of the height of the instrument axis above the station, a correction known as the axis signal correction or eye and object correction is applied
• Correction for curvature
  • Earth has actually a curved surface which is assumed to be a level surface for simplification in the surveying. So the level line between the stations is not horizontal in real conditions but the line of sight by the leveling instrument is horizontal.
  • The vertical distance between the horizontal line and the level line denotes the effect of the curvature of the earth. The correction applied to counter this error is curvature correction
• Correction for refraction
  • The rays of light passing through the atmosphere of different densities bend down. It results in this type of correction.
  • The effect of refraction is 1/7th time the curvature correction but is of the opposite nature. Hence the correction for refraction is additive to the staff reading.

Hence, Correction for sag is the incorrect correction to be applied while determining the difference in elevation between two points, when the observations are from only one point.

There are 4 ways to perform levelling. They are direct levelling, trigonometric levelling, barometric levelling and stadia levelling.

Given data,
The reduced level of formation bed at Zero m chainage = 107 m
A downward gradient = 1 in 100
Hence, the reduced level of formation bed at 1000 m chainage is

In trigonometric levelling, the horizontal distance and vertical angle are measured from the station to the object. It uses trigonometric relations to compute the vertical height of a point.

For leveling work, the following equation must be satisfied:

• ΣBS - ΣFS = last station RL - First station RL
• ΣBS -ΣFS = Σrise -Σfall

Based on the above two equations, the third equation can be obtained.
Last station RL - First station RL = Σrise -Σfall
Note:

• If the Last RL is more than that of the First RL, this means that the ground slope is upward or the ground is rising from the initial benchmark.
• Backsight is a first reading of staff that remains unchanged when the leveling apparatus is taken to another or new point. It is taken on the known reduced level or benchmark.
• Foresight is the reading of the staff or leveling rod which is taken in the forward direction of the leveling process, and it is the last reading of the whole surveying process.
• All the reading which is taken between backsight and foresight is called inter-sight.
• Rise: Staff reading less than the previous reading.
• Fall: Staff reading more than the previous reading.

In differential levelling, a number of inter-stations are located and then the instrument is shifted to each station and the elevation is observed.