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Basics of Geometry

A Bench Mark ...

Question

A Bench Mark (BM) with Reduced Level (RL) = 155.305 m has been established at the floor of a room. It is required to find out the RL of the underside of the roof (R) of the room using Spirit Levelling. The Back Sight (BS) to the BM has been observed as 1.500m whereas the Fore Sight (FS) to R has been observed as 0.575 m (Staff held inverted). The RL(m) of R will be

A

157.860

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B

157.380

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C

155.880

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D

156.230

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Solution

The correct option is B 157.380
In case of inverted staff,

RL of R = Height of instrument + FS

= (RL of benchmark+BS) + FS

$= 155.305 + 1.5 + 0.575$

$= 157.38 m$

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