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A bench Mark has been established at the soffit of an ornamental arch at the known elevation of 100.0 m above sea level. The back sight used to establish height of instrument is an inverted staff reading of 2.105 m. A forward sight reading with normally held staff of 1.105 m is taken on a recently constructed plinth. The elevation of the plinth is

1. 103.210 m
2. 101.000 m
3. 99.000 m
4. 96.790 m

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Correct Answer - Option 4 : 96.790 m

Concept:

The height of the instrument is first established by means of taking sight towards the benchmark of known elevation.

  1. Height of instrument = R.L of bench mark + Back sight reading
  2. Elevation of station = Height of instrument - Fore sight reading

Calculation:

Given:

R.L of benchmark = 100.0 m, Back sight reading = - 2.105 m (staff is inverted)

Fore sight reading = 1.105

Height of instrument = 100 + (-2.105) = 97.895 m

Elevation of plinth = 97.895 - 1.105 = 96.790 m.

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