Levelling terminology and field procedure: Which of the following statements about sights (back sight, fore sight, and staff observations) in levelling is/are correct?

Introduction / Context:
Accurate levelling depends on using the correct terms and sequence of observations. Distinguishing between back sights (BS), fore sights (FS), and intermediate sights (IS) helps structure the field book and provides arithmetic checks for reduced levels (RLs).

Given Data / Assumptions:

• The level is properly set up and the line of collimation is horizontal.
• Staff readings are taken in the order required by the route and change points (CPs).
• Standard rise-and-fall or height-of-collimation methods are used for RL computation.

Concept / Approach:
A sight is any staff observation taken through the instrument. The first reading after a new set-up is the back sight, used to establish the instrument height (HI). At a change point, the final reading from the old set-up is a fore sight; then, after moving the instrument, the first reading on the same CP becomes the new back sight, ensuring continuity of RLs across set-ups.

Step-by-Step Solution:

Verification / Alternative check:
Field-book checks: ΣBS − ΣFS = RL_last − RL_first confirms correct classification and arithmetic integrity of entries.

Why Other Options Are Wrong:

• Each individual statement is correct but partial; the comprehensive correct choice is that all listed statements are true.

Common Pitfalls:
Mislabeling a reading at a CP; mixing IS with FS; forgetting to carry out the BS−FS arithmetic check that reveals transcription errors.

Final Answer:
All of the above