Corrected and true bearings, 5°E declination

Line CD has FB − BB = 180°00′, so stations C and D are free. A and B are suspected (from DA and BC). Declination is 5° East, so true bearing = magnetic bearing + 5°.

Local attraction at a station is detected by comparing the fore bearing (FB) and back bearing (BB) of each line: if there is no local attraction at either end, $\text{FB} - \text{BB} = 180^\circ$ exactly.

Step 1: Check each line

Line CD has a difference of exactly 180°, so stations C and D are free from local attraction. The other stations are corrected one after another from these.

Step 2: Correct the stations in turn

• Line DA: FB at D = 290°30′ is correct. Correct BB should be 110°30′, observed 110°10′, so the correction at A is +0°20′.
• Line AB: FB at A = 80°45′ with correction +0°20′ gives 81°05′. Correct BB should be 261°05′, observed 260°00′, so the correction at B is +1°05′.

Stations affected by local attraction: A, B.

Step 3: Corrected bearings

Check: corrected FB − corrected BB = 180° for every line.

Answer: Corrected bearings (FB): AB = 81°05′, BC = 131°35′, CD = 240°15′, DA = 290°30′. True bearings (FB): AB = 86°05′, BC = 136°35′, CD = 245°15′, DA = 295°30′.