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