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Back Sight


In levelling, a back sight is taken immediately after the optical level is moved to a new location

Technical Information

When surveying a concrete floor by levelling, after all grid points within range of the optical level have been measured, it is necessary to move the optical level to a new location. There is a chance that, in its new location, the optical level will be higher or lower than it was in its old location. 

Therefore, it is necessary to:

  1. Take a measurement (fore sight) from a point between the old and new location of the optical level before it is moved.
  2. Take a measurement (back sight) from the same point once the optical level is set up in its new position.
  3. Calculate the difference in elevation between the old and new optical level setup.


  1. California Department for Transportation: Surveys Manual; chapter 8 


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