Question

If the R.L of a B.M is 50 m, the back sight is 1.25 and foresight is 1.85, then what will be the R.L of the forward station? (in 'm')

1. 46.9
2. 49.4
3. 50.6
4. 53.1

Answer 1

Correct Answer - Option 2 : 49.4

Concept: 

Height of instrument (HI) = Benchmark (BM) + Back sight

Reduced Level of forward station = HI – Foresight Reading

Calculations:

Given,

Reduced level (RL) and Benchmark (BM) = 50 m

Back sight reading = 1.25 m

Foresight reading = 1.85 m

We know that,

Height of instrument (HI) = Benchmark (BM) + Back sight

HI = 50 + 1.25

= 51.25 m

Reduced Level of forward station = HI – Foresight Reading

 R.L = 51.25 – 1.85

 R.L = 49.40 m

∴ Reduced level of Forward station = 49.40 m