In an old plan, a line was drawn to a magnetic bearing of 7° 25' with a magnetic declination of 2° 5' west. If the present magnetic declination is 9° 40' west, then the new magnetic bearing should be drawn at
- A4° 20'
- B-15° 0'Correct
- C-4° 20'
- D15° 0'
Solution
Concept
True bearing stays the same. Magnetic bearing changes when the declination changes.
Formula
True Bearing = Magnetic Bearing + Declination (E) or − Declination (W)
Step-by-step solution
1. Old MB = 7° 25', old declination = 2° 5' W. 2. True bearing = 7° 25' − 2° 5' = 5° 20'. 3. Present declination = 9° 40' W. 4. New MB = TB + 9° 40' = 5° 20' + 9° 40' = 15° 0'.
Explanation
First find the true bearing using the old declination. The true bearing does not change. Then add the new west declination to get the new magnetic bearing, which is 15°.
Shortcut
New MB = Old MB + (new W declination − old W declination) = 7° 25' + 7° 35' = 15°.
Common mistake
Adding the west declination instead of subtracting when finding the true bearing.
Correct answer: B) -15° 0'