Question

Given below are five statements about levelling:
(A) In levelling, datum surface is an arbitrary surface with reference to which the elevations of the points are measured and compared.
(B) The line of collimation or the line of sight is the line joining the intersection of the cross-hairs to the optical centre of the object glass and its continuation.
(C) A foresight is the first staff reading taken after setting up the instrument in any position.
(D) The height of instrument is the elevation of the plane of collimation when the instrument is levelled.
(E) A change point indicates the shifting of the instrument, both back sight and intermediate sight are taken on this change point.
Choose the correct answer from the options given below:

• A. (A), (B) and (D) only.
• B. (A), (B) and (C) only.
• C. (C), (D) and (E) only.
• D. (B), (C) and (D) only.

Hint:

Recall the exact order of readings at one instrument station: back sight first, foresight last, and what a change point connects.

Answer 1

The Correct Option is A

Step 1: Go through the five definitions and match each against the standard surveying terms: datum, line of collimation, backsight/foresight, height of instrument, and change point.
Step 2: (A) and (B) are textbook definitions of datum surface and line of collimation with nothing altered, so both stand.
Step 3: For (C), remember the order of readings at one instrument setup: the very first reading is always the $\text{back sight}$, and only the last reading before moving is the $\text{fore sight}$. The statement swaps these, so (C) fails.
Step 4: For (D), the height of instrument is indeed the reduced level of the line of collimation when the instrument stands level, so (D) holds.
Step 5: For (E), a change (turning) point carries a fore sight from the old setup and a back sight from the new setup, not an intermediate sight, so (E) fails.
\[\boxed{\text{Answer: (A), (B) and (D) only}}\]