Levelling operations – coordinating instrument and staff movement During standard differential levelling in fieldwork, which of the following statements are correct?

Introduction / Context:
Differential levelling relies on precise backsight and foresight observations taken on stable points so that height differences are accurate. Coordination between the instrument operator and the staff holder is therefore critical. This question tests your understanding of best practice during instrument shifts and staff movements to avoid blunders in reduced levels.

Given Data / Assumptions:

• A change point is a firm point used to transfer the line of collimation between setups.
• Backsight (BS) and foresight (FS) must refer to the same physical point during an instrument shift.
• Readings are invalid if either the staff or the instrument is moved at the wrong moment.

Concept / Approach:

The essence of levelling is that the BS and FS defining a change must be observed on a staff kept immovable until both shots are completed. Similarly, while a staff is walked forward, the instrument must remain stationary so that the reading corresponds to a fixed line of sight. Any breach introduces errors in the height of collimation or in the computed reduced level at subsequent stations.

Step-by-Step Solution:

Verification / Alternative check:

Standard levelling procedures emphasize stability at the change point and prohibit simultaneous movements of instrument and staff. Field check notes and arithmetic (summing BS and FS) corroborate correctness when these rules are followed.

Why Other Options Are Wrong:

Neither (a) nor (b): Conflicts with fundamental practice and would cause inconsistencies.

Common Pitfalls:

Moving the staff prematurely; failing to call out “staff fixed” before shifting; forgetting to re-center and re-level the instrument before taking the new BS.

Final Answer:

Both (a) and (b)