Section 4: Soil and Water Conservation Engineering
Surveying and Levelling
Subtopic: Types of levelling
Question Statement:
When precise differential levelling is carried out across a wide river gorge using the reciprocal levelling technique, which of the following systematic instrumental and atmospheric errors are completely eliminated?
Options:
(A) Error due to Earth's curvature
(B) Error due to atmospheric refraction (assuming refraction index remains unchanged during the observations)
(C) Error due to non-parallelism of the line of collimation and the axis of the bubble tube (collimation error)
(D) Error caused by improper verticality or unequal graduation of the levelling staffs
Answer Key:
A, B, C
Step-by-Step Solution & Explanation
- Statements A, B, and C are correct: By taking observations from both banks and taking the mean difference in elevation ($h = \frac{(a_1 - b_1) + (a_2 - b_2)}{2}$), errors due to curvature, steady refraction, and collimation angle cancel out identically.
- Statement D is incorrect: Reciprocal levelling cannot compensate for staff graduation faults or non-vertical holding of the staff.