Correct Answer - Option 4 : 48°45’
Concept:
Local Attraction:
- Local attraction is the amount of deviation of the magnetic needle of the compass under the influence of the external attractive forces caused due to the presence of magnetic materials, such as iron pipes, etc.
- If the difference between the fore and back bearings of the line is 180°, both the end stations are considered to be free from local attraction, provided the compass is devoid of any instrumental errors.
- If the difference is not 180°, it is suggesting presence of local attraction at either or both of the stations.
Calculation:
Difference between back bearing (BB) and fore bearing (FB) for all the stations is shown in the following table:
|
Line
|
Fore Bearing
|
Back Bearing
|
Value of difference between BB and FB
|
|
AB
|
126°45’
|
308°00’
|
181°15’
|
|
BC
|
49°15’
|
227°30’
|
178°15’
|
|
CD
|
340°30’
|
161°45’
|
178°45’
|
|
DE
|
258°30’
|
78°30’
|
180°00’
|
|
EA
|
212°30’
|
31°45’
|
180°45’
|
As, the difference between BB and FB of line DE is exactly 180°, so Station D and E are free from local attraction.
So, now accounting the error for each station one by one in the order of A → B → C.
Correction for Station A:
FB of line EA = 212°30' which is correct. Hence, correct BB of line EA = 212°30' - 180° = 32°30'
∴ Error at A = 31°45' - 32°30' = - 0°45'. Hence correction at A = +0° 45'.
Correction for Station B:
Similarly, correct FB of line AB = 126°45' + 0° 45' = 127°30'
Hence, correct BB of AB = 127°30 + 180° = 307° 30'.
∴ Error at B = 308° - 307° 30' = + 0° 30'; Hence correction at B = -0° 30'
Hence correct bearing of BC = 49°15’ - 0° 30' = 48° 45'
