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Q: Identify the primary cause of local attraction in compass surveying?
  • A. Instrumental errors in the compass
  • B. Nearby magnetic substances or electrical interference
  • C. Natural Earth's magnetic field
  • D. Human errors during observation
Correct Answer: Option B - Detention of local attraction– • The local attraction at any station is detected by observing the fore and back bearing of the line. • If the difference between them is 1800 both the end stations are considered to be free from local attraction provided the compass is not having any instrument errors. • If the differences is not 1800, the discrepancy introduced may be because of : (i) An error is observation of either fore or back bearing, or both. (ii) Presence of local attraction at either or both of the stations.
B. Detention of local attraction– • The local attraction at any station is detected by observing the fore and back bearing of the line. • If the difference between them is 1800 both the end stations are considered to be free from local attraction provided the compass is not having any instrument errors. • If the differences is not 1800, the discrepancy introduced may be because of : (i) An error is observation of either fore or back bearing, or both. (ii) Presence of local attraction at either or both of the stations.

Explanations:

Detention of local attraction– • The local attraction at any station is detected by observing the fore and back bearing of the line. • If the difference between them is 1800 both the end stations are considered to be free from local attraction provided the compass is not having any instrument errors. • If the differences is not 1800, the discrepancy introduced may be because of : (i) An error is observation of either fore or back bearing, or both. (ii) Presence of local attraction at either or both of the stations.