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Repetition method

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Repetition method is the method of measuring horizontal angles with more precision and accuracy. In this method, the same angle is measured 6 times. 3 each with face left and face right (face left means when horizontal circle is on the left of observer and face right implies when horizontal circle is on right of observer), the arithmetic mean of these observations gives true value of an angle. Also when the precision of the instrument is desired to be more than the least count of the instrument, this method is used.[1]

Example

Consider horizontal angle AOB, where O is the station point.The steps followed to determine horizontal angle AOB by repetition method can be summarized in below mentioned steps:

Procedure

  • Firstly the theodolite is mounted on tripod stand with its proper adjustments.
  • The theodolite is then centered over the selected station O and is properly levelled using its foot screws and the plate bubble is brought to the centre of the run. Then using its focusing screws the points A and B are properly sighted by telescope. Centering, levelling and focusing of theodolite are called as temporary adjustments of theodolite.
  • The theodolite is set on the face left mode and vernier A is set on 0 by making it coincide with 0 of main scale with the help of upper clamp and its tangent screw and station A is sighted by using lower clamp and its tangent screw.[2] Also check whether vernier A still reads 0.[3]
  • Now unclamp the upper clamp and rotate the instrument towards B in clockwise manner until it is accurately bisected with the help of upper tangent screw. Note the readings of verniers A and B to get the value of angle AOB. Vernier A gives value of angle AOB directly while Vernier B gives value of angle by deducing its reading by 180.
  • Tight the upper clamp so that the vernier reading does not change and loose the lower clamp and turn the telescope clockwise to sight A again. Bisect A accurately by using lower tangent screw. Tight the lower clamp and unclamp the upper clamp, turn the telescope clockwise and sight B. Bisect B accurately using upper tangent screw.
  • Repeat the process until angle is measured required number of times usually 3.The average angle will be the arthemetic mean of total angle measured.
  • Same procedure can be used while computing values for face right mode.
  • The true value of horizontal angle is then computed by taking average of the angles obtained with face left and face right.[4]

Conclusion

Repetition method is used when accuracy is must. For rough or approximate survey work ordinary method of measuring horizontal angles is preferred as it is less time consuming.[5]

See also

References

  1. ^ National Programme on Technology Enhanced Learning official website. http://nptel.ac.in/courses/105107122/modules/module6/html/73.htm. Retrieved on 6 September 2014
  2. ^ Civil Engineering hub. http://www.civilengineeringnews.tk
  3. ^ A text book of surveying by Dr. C L Kochher. Retrieved in August 2014
  4. ^ Surveying Volume 1 by Dr. B.C Punmia, Dr. Ashok Kumar Jain and Dr. Arun Kumar Jain. published by Laxmi publications New Delhi. Retrieved on 6 September 2014
  5. ^ Surveying Volume 1 by Dr. B.C Punmia, Dr. Ashok Kumar Jain and Dr. Arun Kumar Jain. published by Laxmi publications New Delhi. Retrieved on 6 September 2014
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