If the equality of back sight distance and foresight cannot be maintained, under such condition, which one of the following levelling types is most important part of geodetic surveying ?
- ASpirit levelling
- BReciprocal levellingCorrect
- CTrigonometric levelling
- DOrdinary levelling
Solution
Concept
Reciprocal levelling
Step-by-step solution
1. When BS and FS distances cannot be kept equal (for example across a wide river or ravine), reciprocal levelling is used. 2. Readings are taken from both banks and averaged to eliminate the instrumental, curvature and refraction errors.
Explanation
Reciprocal levelling cancels errors due to collimation, curvature and refraction when the sights are unequal.
Why the other options are wrong
Ordinary and spirit levelling require equal sight distances. Trigonometric levelling is used where direct levelling is impossible, but it does not address unequal sights.
Shortcut
Unequal sights over an obstacle → reciprocal levelling.
Common mistake
Choosing trigonometric levelling.
Correct answer: B) Reciprocal levelling