Instrumental Errors in Surveying

  • The main source of instrumental error is residual collimation error. Keeping the horizontal lengths of the backsights and foresights at each instrument position equal will cancel this error. Where the observational distances are unequal, the error will be proportional to the difference in distances. The easiest approach to equalizing the sight distances is to pace from backsight to instrument and then set up the foresight change point the same number of paces away from the instrument.
  • Parallax error.
  • Staff graduation errors may result from wear and tear or repairs and the staffs should be checked against a steel tape. Zero error of the staff, caused by excessive wear of the base, will cancel out on backsight and foresight differences. However, if two staffs are used, errors will result unless calibration corrections are applied.
  • In the case of the tripod, loose fixings will cause twisting and movement of the tripod head. Overtight fixings make it difficult to open out the tripod correctly. Loose tripod shoes will also result in unstable set-ups.

 

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