For a station to be free from local attraction,
- AFore and back bearing of line should be measured accurately
- BFore and back bearing of line should differ by exactly 180°Correct
- CFore and back bearing of line should be exactly same but with opposite sign
- DFore and back bearing of line should differ by exactly 90°
Solution
Concept
Detection of local attraction
Formula
Fore bearing − Back bearing = 180° (no local attraction)
Step-by-step solution
1. If both stations are free from local attraction, the fore bearing and back bearing of a line differ by exactly 180°. 2. If not, local attraction is present at one or both stations.
Explanation
A station is free from local attraction when the observed fore bearing and back bearing of the line differ by exactly 180°. A difference other than 180° shows local attraction at one or both ends.
Shortcut
FB − BB = 180° → no local attraction at either end.
Common mistake
Thinking that accurate observation removes the effect.
Correct answer: B) Fore and back bearing of line should differ by exactly 180°