Why is it important to balance the foresight and backsight distances in levelling?
- ATo eliminate the need for applying curvature and refraction corrections
- BTo reduce errors due to Earth’s curvature and atmospheric refraction
- CTo avoid errors caused by the inclined line of collimationCorrect
- DTo avoid change points
Solution
Concept
Balancing backsight and foresight distances in levelling
Formula
NONE
Step-by-step solution
1. If the line of sight of the telescope is not perfectly horizontal, an inclination error occurs in staff readings. 2. Keeping the backsight and foresight distances equal ensures that equal error is introduced into both readings, which cancels out when taking their difference. 3. Hence, balancing distances avoids errors caused by an inclined line of collimation. Answer: C
Explanation
If the line of collimation in a leveling instrument is not perfectly parallel to the axis of the bubble tube, it creates an angular collimation error that affects staff readings. By keeping the backsight and foresight distances equal from the instrument, equal error amounts are introduced into both readings, which cancel out when calculating elevation differences. Therefore, balancing sight distances effectively eliminates errors caused by an inclined line of collimation, making option C correct.
Why the other options are wrong
A) Balancing sight distances eliminates collimation error, not the physical occurrence of curvature/refraction. B) Equalizing sight distances eliminates collimation error rather than directly reducing atmospheric refraction. D) Balancing sight distances has no relation to avoiding change points in a level run.
Shortcut
Equal BS and FS distances eliminate curvature, refraction, and collimation errors.
Common mistake
Selecting option B instead of C by assuming balancing sight distances primarily eliminates curvature and refraction errors rather than line of collimation error.
Correct answer: C) To avoid errors caused by the inclined line of collimation