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Both the end stations are free from local attraction if the difference between fore and back bearing of a line is:
1. 0° 
2. 90° 
3. 180° 
4. 270° 

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Correct Answer - Option 3 : 180° 

Concept:

Local attraction:

  • While compass surveying, the magnetic needle is sometimes disturbed from its normal position under the influence of external attractive forces. Such a disturbing influence is called a local attraction.
  • It is the attraction of the magnetic needle to a local magnetic field other than the earth's magnetic field.
  • It can be caused by iron pipes, steel pipes, vehicles, etc.
  • If the difference between fore bearing and back bearing is 1800 then both the end stations are considered to be free from local attraction

Back bearing:

  • The bearing of a line measured in the backward direction or opposite to the direction of progress of survey is known as a back bearing.
  • It is the first reading taken at instrument position.

Fore bearing:

  • The bearing of a line measured in the forward direction or along the progress of the survey is known as a fore bearing.
  • It is the last reading taken at instrument position after it either end of work or instrument position is changed. 
  • Fore bearing = Back bearing ± 180°

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