Levelling procedure: at any new instrument setup, what is the very first staff reading taken from that level station called?

Introduction / Context:
Levelling notes follow a strict sequence to ensure consistent calculation of the height of instrument (H.I.) and reduced levels (R.L.). The naming of the first observation at each setup is standardized across field practice.

Given Data / Assumptions:

• Instrument is set up and levelled at a new station.
• A known or previously computed R.L. is available (bench mark or turning point).
• Readings are recorded in B.S., I.S., F.S. columns.

Concept / Approach:

The first reading from a new setup is a back-sight on a point of known R.L., allowing computation of H.I. via H.I. = R.L. + B.S. Subsequent readings may be intermediate sights to determine unknown R.L.s, and the last reading before moving is the fore-sight on a change point or the final point.

Step-by-Step Solution:

Verification / Alternative check:

The arithmetic check ΣB.S. − ΣF.S. = R.L.(last) − R.L.(first) relies on this convention.

Why Other Options Are Wrong:

F.S. is the last reading from a setup; I.S. is neither first nor last; “any sight” is not acceptable in standard field books.

Common Pitfalls:

Recording a turning-point reading in the wrong column; forgetting to start with a B.S., which breaks the check.

Final Answer:

Back-sight (B.S.)