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To find the Reduced Level of a roof slab of building, staff readings were taken from a particular set-up of the levelling instrument. The readings were 1.050 m with staff on the Bench Mark and 2.300 m with staff below the roof slab and inverted.

Taking the Reduced Level of the Bench Mark as 135.15 m, the Reduced Level of the roof slab will be?


1. 129.800
2. 131.900
3. 134.400
4. 138.500

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Best answer
Correct Answer - Option 4 : 138.500

Concept:

HI = RL + BS

RL of roof slab = HI – FS

Where, 

BS = Back sight

FS = Fore sight

HI = Height of instrument

RL = Reduced level

Calculation:

Given,

BS = 1.050 m

RL = 135.15 m

HI of instrument = RL + BS

= 135.15 + 1.050

= 136.2 m

FS = -2.300 m (as inverted)

RL of roof slab = HI – FS

= 136.2 – (-2.300)

= 138.5 m

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