Question

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° 

Answer 1

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°