Question

(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:

Watch for two statements that quietly swap the standard meanings of backsight, foresight and intermediate sight.

Answer 1

The Correct Option is A

Step 1: Work through each surveying definition given in (A) to (E) and compare it against the standard terminology used in levelling.
Step 2: Statement (A) defines the datum surface as an arbitrary reference surface against which point elevations are measured and compared. This is exactly the standard textbook definition of a datum, so (A) is correct.
Step 3: Statement (B) defines the line of collimation (line of sight) as the line joining the intersection of the cross-hairs to the optical centre of the object glass, continued onward. This is also the accepted definition, so (B) is correct.
Step 4: Statement (C) claims a foresight is the first staff reading taken after setting up the instrument at any position. This is actually the definition of a BACKSIGHT, which is the first reading taken on a staff held at a point of known elevation right after the instrument is set up, used to find the height of instrument. A foresight, by contrast, is the LAST reading taken in that setup, just before the instrument is shifted to a new position. Since the statement mixes up foresight with backsight, (C) is incorrect.
Step 5: Statement (D) defines the height of instrument as the elevation of the plane of collimation once the instrument is levelled. This matches the accepted definition, so (D) is correct.
Step 6: Statement (E) claims a change point is where both a backsight and an intermediate sight are taken. In reality, a change point (turning point) is where a FORESIGHT is taken to close out the old instrument position and a BACKSIGHT is taken after shifting to the new position, not an intermediate sight. Because the statement swaps in "intermediate sight" instead of "foresight," (E) is incorrect.
Step 7: The statements that hold up are (A), (B) and (D), which corresponds to option 1.