Errors in Theodolite

Types of Errors in Theodolite

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Theodolite may be defined as the optical survey instrument that is used to measure the angles between the specified points both in the vertical and horizontal plane. Some errors in theodolite may be encountered which are explained below.

In other words, theodolite may be defined as the instrument fitted with a small telescope that can move freely in the horizontal and vertical plane so as to measure the horizontal and vertical angles in surveying.

It is extensively used in traversing, land surveying and other infrastructure construction. However, if not handled properly or sufficient care is not taken, certain errors may be encountered while using the theodolite. These errors have been described in brief below.

 

Errors In Theodolite

There are 3 types of main errors in theodolite. They are:

 

1. Instrumental Errors

The instrumental errors are the type of errors that arise on account of the defect or malfunction of the theodolite itself.

The following types of instrumental errors may occur while using the theodolite:

 

a. Non-Adjustment of Plate Levels

When the plate levels of the theodolite are not perpendicular to the vertical axis, the vertical axis is not perfectly vertical which results in the inclination of the horizontal circle.

Due to the inclined horizontal circle, the angles are measured in an inclined plane which should have actually been measured in the horizontal plane itself thereby causing the error.

This type of error can be eliminated by levelling the theodolite with proper reference to the altitude bubble of the theodolite.

 

b. Non-Perpendicularity of Line of Collimation to the Horizontal Axis

When the line of collimation is not truly perpendicular to the horizontal axis, during the revolution of the telescope; it will result in the tracing out of the surface of a cone in place of a plane.

Due to this, the measurements of the horizontal angles taken between points that are at greatly varied elevations will thus be incorrect.

This type of error in theodolite can be eliminated by reading angles on both the faces of the theodolite and thus noting the mean value of the two observed values.

 

c. Non-Perpendicularity of the Horizontal Axis to the Vertical Axis

When the horizontal axis of the theodolite is not truly perpendicular to the vertical axis, during the lowering or raising of the telescope, the line of collimation will not revolve in the vertical plane.

This gives rise to the angular errors i.e. errors will be encountered both in the horizontal and vertical angular readings.

This type of error in theodolite can be eliminated by reading angles on both the faces of the theodolite and thus noting the mean value of the two observed values.

 

d. Non-Parallelism of the Line of Collimation and the Axis of the Telescope Level

When the line of collimation is not parallel to the telescope level of the theodolite, the zero lines of the vertical verniers does not represent a true line of reference.

This results in the occurrence of angular errors in the measurement of vertical angles.

This type of error can be eliminated by taking two vertical angle readings. One of the angular measurements must be taken with the telescope normal; while the other observation must be taken with the telescope inverted. The mean of the two values must be then taken as the final reading.

 

e. Unequal Graduations of the Circle

Sometimes, the graduations of the circle in the theodolite may not be equal thereby causing the error in measurements.

Such type of error can be reduced by taking the angular measurements many times on different parts of the circle and thus taking the mean of the observed readings as the final observation value.

 

f. Non-Concentricity of the Axes of the Upper and Lower Plates

When the axes of the upper and the lower plates of the theodolite are not concentric, angular errors may be encountered.

The non-concentricity of the axes makes the angular observations of the verniers incorrect.

This type of error can be prevented by reading the observations from both the verniers and taking the average of the two observations as the final reading.

 

g. Eccentric Verniers

When the verniers are eccentric, the zero markings of the verniers will not be opposite to each other diametrically thereby leading to the error in the readings.

This type of error can be prevented by reading the observations from both the verniers and taking the average of the two observations as the final reading.

 

h. Non-Verticality of the Vertical Hair

When the vertical hair of the theodolite telescope is not truly vertical, errors may occur.

Such type of error in theodolite can be reduced by making use of the hair near the horizontal hair for bisecting the signal.

 

2.  Observational Errors 

The observational errors arise on account of the improper handling of the instrument or improper noting of the observations by the observer/ surveyor.

Due to this reason, this type of error is also commonly referred to as the personal error.

The observational errors can be divided into the following:

 

a. Improper or Inaccurate Centering

When the theodolite is not centred correctly, errors may occur.

The error due to the improper centring of the theodolite is one of the most common errors in theodolite.

This type of error varies on the basis of the length of sight.

It can be minimized by proper centring of the theodolite over the station mark.

 

b. Inaccurate Leveling

When the theodolite is not levelled properly, errors may occur. In such a case, the vertical axis of the theodolite is not perfectly vertical which results in the inclination of the horizontal circle. Due to the inclined horizontal circle, the angles are measured in an inclined plane which should have actually been measured in the horizontal plane itself thereby causing the error.

This type of error can be reduced by the proper levelling of the instrument.

 

c. Use of Wrong Tangent Screw

Errors in theodolite may occur when the surveyor or the observer makes use of the wrong tangent screw.

Usually, this type of error is common in inexperienced observers.

Such type of error can be eliminated if proper care is taken while using the screws.

 

d. Slip

When the theodolite is not screwed tightly to the tripod or when the clamping of the shifting head or the lower head is not done firmly, errors may occur.

Such error can be prevented by the proper clamping of the heads and firm tightening of the clamps of the theodolite.

 

3. Natural Errors

Sometimes, certain errors may occur due to natural causes. This type of errors includes the following:

a. Errors due to vibration of the theodolite in windy areas.

b. Errors due to heat and sun.

c. Errors due to high temperature leading to irregular refraction.

 

Read More: Errors in Levelling

Read More: Total Station

 

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