Levelling field procedure: Identify the correct statement about the sequence of sights on change points and at the start/close of a levelling line.

Introduction / Context:
Levelling book work follows a strict sequence to avoid blunders. Understanding which readings are back sights (B.S.) and fore sights (F.S.) at the start, at change points (turning points), and at the close of a run ensures that checks like ΣB.S. − ΣF.S. match the elevation differences.

Given Data / Assumptions:

• A levelling line starts on a benchmark (known R.L.).
• At change points, the instrument is moved and the same point is sighted from both setups.
• The run ends at a final point or benchmark.

Concept / Approach:

By convention, the first staff reading taken after setting up is a back sight to establish the height of instrument (H.I.). The last reading before moving the instrument from its current setup is a fore sight to a change point. After moving, the first reading is again a back sight on that change point. A levelling line, therefore, commences with a back sight and must close with a fore sight to the final point.

Step-by-Step Solution:

Verification / Alternative check:

The arithmetic check ΣB.S. − ΣF.S. = R.L.(last) − R.L.(first) is satisfied only when the line starts with B.S. and ends with F.S., aligning with this convention.

Why Other Options Are Wrong:

Option A can be ambiguously interpreted; the safe universally correct statement about the line is (d). Options B and C contradict standard procedure.

Common Pitfalls:

Mislabeling a C.P. reading; forgetting to close with an F.S.; mixing up the sequence which breaks the standard book check.

Final Answer:

The line commences with a back sight and closes with a fore sight