When the difference in elevation between two points is determined from a single setup by back sighting on one point and fore sighting on the other point and lengths of foresight and backsight are made equal then
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Only the error due to non-parallelism of line of collimation and axis of bubble tube when bubble is central, is eliminated
Only errors due to curvature and refraction are eliminated
Errors due to non-parallelism of line of collimation and axis of bubble tube when bubble is central and errors due to curvature and refraction are eliminated
No errors are eliminated
Errors due to non-parallelism of line of collimation and axis of bubble tube when bubble is central and errors due to curvature and refraction are eliminated
Balancing Backsights and Foresights: When the difference in elevation between any two points is determined from a single set-up by backsighting on one point and foresighting on the other, the error due to non-parallelism of line of collimation and axis of the bubble tube (when the bubble is in the centre of the run) and also the error due to curvature and refraction may be eliminated if the lengths of two sights can be made equal.
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