Section 4: Soil and Water Conservation Engineering
Surveying and Levelling
Subtopic: Types of levelling
Question Statement:
The back sight of 1.258 m was observed for the bench mark (BM) at reduced level (RL) of 48 m. The corresponding fore sight on the staff held vertically inverted to the underside of a bridge beam is 4.645 m. The RL at the underside of the bridge beam in m is
Options:
(A) 44.613
(B) 46.936
(C) 51.581
(D) 53.903
Official Answer Key:
D
Step-by-Step Solution & Explanation
Height of instrument HI = RL(BM) + BS = 48 + 1.258 = 49.258 m. For an inverted staff reading (used for overhead points such as the underside of a beam/soffit), the fore-sight reading is added to the HI rather than subtracted, since the staff graduations increase downward when inverted: RL(beam) = HI + FS = 49.258 + 4.645 = 53.903 m → option (D). Subtracting FS (the normal staff convention, giving 44.613, option A) is the classic distractor for missing the inverted-staff sign reversal.
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