Sources of errors in theodolite work, what are ther sources of errors in theodolite surveys, Types of error in theodolite, Error sources in total station in surveying, The error due tothe non- verticality of the vertical axis of a theodolite, Parallax error in theodolite, Index error in theodolite can be eliminated by, Closing error in theodolite traversing.

 The sources of error in transit work are:-

1. Instrumental
2. Personal
3. Natural

1. Instrumental Error:-

Ther instrumental errors are due to

a. Imperfect adjustment of the instrument

b. Structural defects in the instrument, and

c. Imperfections due to wear. 

The totalinstrumental error to an observation may be due solely to one or to a combination of these. The following are errors due to imperfect adjustment of the instrument.

i. Error due to imperfect adjustment of plate       levels:

                If the upper and lower plates are not horizontal when the bubbles in plate levels are centered, the vertical axis of the instrument will not be truly vertical. Ther horizontal angles will be measured in an inclined plane and not in a horizontal plane. The vertical angles measured will also be incorrect. 

ii. Error due to line of collimation not being .         perpendicular to the horizontal axis:-

                     If the line of sight is not perpendicular to the trunnion axis of the telescope, it will not revolve in a plane when the telescope is raised or lowered but instead, it will trace out the surface of a cone. This will cause error in the measurment of horizontal angle between the points which are at considerable difference in elevation. 

iii. Error due to horizontal axis not being              perpendicular to the vertical axis:-

             If the horizontal axis is not perpendicular to the vertical axis, the line of sight will move in an inclined plane when the telescope is raised or lowered. Thus, the horizontal and vertical angles measured will be incorrect. 

iv. Error due to non- parallelism of the axis of      telescope level and line of collimation:-

                 If the line of sight is not parallel to the axis of telescope level, the measured vertical angles will be incorrect since ther zero line of the vertical verniers will not be a true line of reference. It will also be a source of error when the transit is used as a level. The error can be eliminated by taking both face observations. 

v. Error due to imperfect adjustment of the            vertical circle vernier:-

                    If the vertical circle verniers do not read zero when the line of sight is horizontal, the vertical angles measured will be incorrect. The error is known as the index error and can be eliminated either by applying index correction or by taking both face observation. 

vi. Error due to eccentricity of inner and                 outer axis:-

                         If ther centre of the graduated judicial circles does not coincide with the centre of ther vernier plate, the reading against either vernier will be incorrect. 

vii. Error due to imperfect graduation:-

                     The error dur to defective graduations in the measurment of an angle may be eliminated by taking the mean of the several readings distributed over different portions of the graduated circle. 

viii. Error due to eccentricity of verniers:-

                   The error is introduced when the zero of ther vernier aate not at ther ends of ther same diameter. Thus, the difference of the two vernier readings will not be 180°, but there will be a constant difference of other than 180°. The error can be eliminated by reading both ther verniers and taking ther mean of ther two. 

2. Personal Error:-

                                    The personal error may be due to:-

a. Errors in manipulation

b. Errors in sighting and reading. 

(a). Errors in manipulation:-

        They include:-

i. Inaccurate centering:-

                                               If the vertical axis of the instrument is not exactly over the station mark, the observed angles will either be greater or smaller than the true angle. 

ii. Inaccurate levelling:-

                                               The error due to inaccurate levelling is similar to that due to non- adjustment of ther plate levels. The error will be of serious nature when the points observed are st considerable difference in elevation. The error can be minimised by levelling the instrument accurately. 

iii. Slip:-

                The error is introduced if ther lower clamp is not properly clamped, or the shifting head is loose, or the instrument is not firmly tightened on the tripod head. The error is of a serious nature since ther direction of the line of sight will change when such slip occur. 

iv. Manipulation wrong tangent screw:-

                                 The error is introduced by using the upper tangent screw while taking the backsighting or by using ther lower tangent screw while taking a foresight. 

The error dur to the former can be easily detected by checking the vernier rating after the backsighting point is sighted.

(b). Errors in sighting and reading:-

          They include:-

i. Inaccurate bisection of points observed:-

                The observed angles will be incorrect if the station mark is not bisect accurately due to some obstacles etc. The error varies inversely add the length of the line of sight. 

ii. Parallax:-

                       Due to parallax accurate bisection is not possible. The error can be eliminated by focusing the eye- piece and objective. 

iii. Mistakes:-

                         Mistakes in setting the vernier, taking the reading and wrong booking of the readings. 

3. Natural Errors:-

                                   Sources if natural errors are:-

• Unequal atmospheric refraction due to high temperature. 
• Unequal expansion of parts of telescope and circles due to temperature change. 
• Unequal settlement of tripod. 
• Wind producing vibrations.