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Which of the following equations is correct?
1. Back bearing = Fore bearing ± 90°
2. Back bearing = Fore bearing ± 180°
3. Back bearing = Fore bearing ± 270°
4. None of the above

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Correct Answer - Option 2 : Back bearing = Fore bearing ± 180°

Concept:

Back bearing (BB) = Fore bearing (FB) ± 180°. +ve sign is used if FB is less than 180° and –ve sign is used if FB is more than 180°.

∴ Back bearing = Fore bearing ± 180°

Important points:

  • Whenever there is any station affected by local attraction, then the difference between the fore bearing and back bearing is not equal to 180°
  • A freely suspended and properly balanced magnetic needle is expected to show magnetic meridian. However, local objects like electric wires and objects of steel attract magnetic needle towards themselves
  • Thus, the needle is forced to show a slightly different direction, this disturbance is called local attraction
  • For detecting local attraction it is necessary to take both fore bearing and back bearing for each line
  • If the difference is exactly 180°, the two stations may be considered as not affected by local attraction
  • If difference is not 180°, better to go back to the previous station and check the fore bearing
  • If that reading is same as earlier, it may be concluded that there is local attraction at one or both stations

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