Levelling is defined as the branch of surveying that is essentially used for determining the relative height of the different points on, above and below the surface of the ground. The basic principle of levelling is to determine the horizontal line of sight with respect to which the vertical distances of the points below or above this line of sight are determined. However, certain errors in levelling may be encountered if proper measures are not adopted.
Types of Errors In Levelling
There are mainly 5 types of errors in levelling. They are:
1. Instrumental Error
The error that arises on account of the defects or certain problems associated with the instruments that are used for the levelling is called Instrumental Error. The instrumental error may be further classified into the following types:
a. Imperfect Adjustment
The error due to the imperfect adjustment of the instrument is one of the most common types of error in levelling.
The levelling equipment must be properly adjusted before the readings are taken.
The temporary adjustment of the instrument is done such that the line of collimation is horizontal.
When the adjustment is done properly, the line of collimation lies exactly parallel to the bubble axis of the instrument such that it becomes perfectly horizontal when the bubble is centred.
On the other hand, when the instrument is not adjusted properly, the line of collimation is inclined even though the bubble is centred. Thus, errors may be encountered.
The errors due to the imperfect adjustment can be eliminated by adopting the following steps:
~ Proper adjustment and testing of the instrument before use.
~ Equalization of the backsight and foresight distances.
b. Defective Level Tube
Sometimes, if the bubble is sluggish, it tends to remain in the centre even when the bubble axis is not perfectly horizontal. The over-sensitive tube may also pose significant difficulty in levelling of the instrument. Hence, it must be checked beforehand that the level tube of the instrument has the required suitable sensitiveness only.
c. Incorrect Graduations of the Staff
The errors may be encountered when the levelling staff is incorrectly graduated.
The graduations of the new staff must be checked utilizing an invar tape to ensure that the graduations are correct.
d. Shaky Tripod
When a shaky tripod is used, it makes the instrument unstable and the readings taken may contain many errors i.e. the possibility of erroneous readings increases.
To prevent this, the tripod must be properly checked and tested before use. If loose joints are present in the tripod, they must be tightened properly.
e. Telescope not Parallel to the Bubble Tube
When the telescope of the instrument is not parallel to the bubble tube, errors may be encountered during levelling. Such an error can be prevented by the permanent adjustment of the instrument.
f. Telescope not at Right Angles to the Vertical Axis
When the telescope of the instrument is not at the right angle to the vertical axis, errors may be encountered during levelling.
Such an error also can be prevented by the permanent adjustment of the instrument.
2. Error of Collimation
The error of Collimation or simply the collimation error is a common type of error in reciprocal levelling.
This type of error occurs when the axis of collimation is not truly horizontal when the instrument is level.
The axis of collimation is tilted concerning the horizontal by a certain angle.
The collimation error can be eliminated by balancing the back sight and the foresight.
3. Errors due to Settlement of Level and Staff
Sometimes, errors may occur when the settlement of the level or the staff occurs. They are further described in brief below.
a. Settlement of Level
The settlement of level is most likely to occur when the levelling has to be done in soft ground.
When the instrument is set up on the soft ground, subsequent settlement of the level may occur while the back sight and the foresight reading are taken. Such settlement makes the fore sight-reading significantly smaller than that it actually must be. To prevent such error, the level must always be set up on the relatively firm and hard ground and the tripod legs must be pressed into the ground tightly.
Wooden stakes can also be used for such purpose. As far as possible, the foresight reading must be taken immediately after the back sight-reading is noted.
b. Settlement of Staff
Like the settlement of the level; settlement of the staff may also occur in the soft grounds particularly during the change point. But, in this case, the backsight will be greater than that it must be. Such type of error can be prevented by avoiding the soft ground as a change point or by driving pegs of steel four-plates that can serve as the temporary change points.
4. Errors of Manipulation
The errors of manipulation include the following:
a. Improper Leveling of the Instrument
Certain errors may arise on account of the careless or improper levelling of the instrument. Proper care must be taken to prevent such error.
b. Non-Centralization of Bubble while Taking the Reading
When the bubble is not at the centre while taking the reading; errors may occur.
It is one of the common types of error in levelling. To prevent such error, the position of the bubble must be checked every time before the reading is taken. The bubble must always be brought at the centre of its run utilizing the levelling screw.
c. Inefficient Removal of the Parallax
The error due to inefficient removal of the parallax is associated with the improper focusing of the eyepiece and the object-glass of the telescope. Such type of error may be eliminated by proper focusing before taking the readings.
d. Non-Vertical Staff
When the staff is not held vertically, errors may arise while taking the readings. In such a case, the readings will be greater than they are. To prevent such error, it must be taken care that the staff is held completely vertical while the reading is being taken. This error can be fixed utilizing a spirit level or pendulum plumb bob.
5. Errors due to Natural Sources
Some errors that may arise due to natural sources are:
a. Curvature and Refraction
The curvature of the earth may cause the objects to appear slightly smaller than they are; while the refraction may cause the objects to appear slightly bigger than they are. However, the error due to curvature and refraction may not be taken into consideration, since it is very small and hence negligible i.e. only 0.003 m for a 200 m sight-length. But, for long sights, the correction for curvature and refraction has to be applied.
During the windy climatic condition, the wind may cause subsequent vibration of the instrument leading to reading errors. In such a case, large readings must not be taken as far as possible.