Instrument testing: The method of reversal is used for what purpose in surveying instrument adjustments and checks?

Introduction / Context:
Accurate surveying depends on specific geometric relationships between instrument axes (line of sight, horizontal axis, vertical axis). The method of reversal is a classical test technique to reveal misalignments and verify perpendicularity or parallelism by observing symmetry under reversed configurations.

Given Data / Assumptions:

• Reversal means interchanging positions (e.g., face left/face right, turning telescope through 180°, swapping ends).
• Correct geometry yields invariant readings under reversal.
• Any difference between the pair of readings indicates an error relationship.

Concept / Approach:

By taking observations in two opposite configurations, systematic geometric errors change sign while true values remain the same. Averaging or comparing the two reveals whether a component is truly parallel/perpendicular to another and makes any erroneous relationship evident. The method is widely used for collimation checks, trunnion axis tests, and bubble line alignment, among others. Hence both statements (a) and (b) accurately describe its purpose and effect.

Step-by-Step Solution:

Verification / Alternative check:

Instrument adjustment procedures in manuals employ reversal for line-of-sight error, index error, and bubble axis alignment, confirming its generality.

Why Other Options Are Wrong:

Restricting to a single error type (e) is too narrow; choosing only (a) or only (b) omits part of the method’s function; “neither” contradicts standard practice.

Common Pitfalls:

Failing to center or level consistently between reversals; not recording both configurations accurately, which obscures diagnostics.

Final Answer:

Both (a) and (b)