Fore Bearing
It is the bearing of line when the first letter of line say AB is taken as origin. This is to be written as Fore Bearing (F.B).
Back Bearing
It is the bearing of line when second letter of line say AB is taken as origin and this is to be written as Back Bearing (B.B).
Theoretical difference between Fore Bearing (F.B) and Back Bearing (B.B) should be 180°.
Local Attraction
If the difference between magnetic Fore Bearing and Back Bearing of a line is not exactly 180°, it may be due to presence of local attraction at one of the both stations. If this difference is exactly 180° then both stations are free from local attraction. Local attraction may be due to following reasons.
- Overhead electrical wires
- Magnetic materials in the vicinity
Practice Problem
In the following table observed Bearings are given, we will compute the corrected bearings and Internal Angles.
| Line | Observed | Correction | Corrected | ||
|---|---|---|---|---|---|
| F.B | B.B | F.B | B.B | ||
| AB | 70° 00′ | 251° 00′ | A= +30′ , B= -30′ | 70° 30′ | 250° 30′ |
| BC | 328° 00′ | 145° 00′ | – | 327° 30′ | 147° 00′ |
| CD | 225° 00′ | 71° 00′ | – | 257° 30′ | 77° 30′ |
| DA | 139° 00′ | 316° 00′ | – | 136° 30′ | 316° 30′ |
By observing the table, it may be noted that no line has a difference of exactly 180° between Fore Bearing and Back Bearing. In such a case, a line where the difference is closest to 180° is selected. Such a line is called line of least disagreement, for this line correction is assign to each of the two stations of that line with opposite sign. In the above table line AB is selected for error distribution. Now, we will compute internal angles from these corrected Bearings.
A = (360° – 316° 30′) + 70° 30′ = 114° 00′
B = 327° 30′ – 250° 30′ = 77° 00′
C = 257° 30′ – 147° 30′ = 110° 00′
D = 136° 30′ – 77° 30′ = 59° 00′
Before computation of internal angles you need to draw a rough sketch of scheme based on corrected bearings so that you can judge which angle is lying in which quadrant.
