FORE AND BACK BEARINGS

In compass surveying, two bearings are observed for the each line, one from each end.
Fore Bearing: The bearing of a line measured in the direction of the progress of survey is called Fore Bearing or Forward Bearing (FB) of a line AB
Back Bearing: The bearing of a line measured in the direction opposite to the survey is called the Back Bearing or Backward Bearing (BB) of a line AB .

RULES TO BE REMEMBERED REGARDING FB AND BB:

1. In the WCB system, the difference between the FB and BB should be exactly 180°,
BB =FB ±180°
Note: Use the positive sign when FB is less than 180°, and the negative sign when it is greater than 180°.
(a) For example, if the FB of AB is 0, then its BB is (0₁ + 180°) Fig.(a).
(b) For example, if the FB of PQ is 02. then its BB is (02- 180°) Fig. (b).

2. In the QB system, the FB and BB are numerically equal, but the quadrants are just opposite.
a) For example, if the FB of AB is S 30° 00′ W. then its BB is N 30° 00 E (Fig (a)).
(b) For example, if the FB of PQ is S 45° 00′ E, then its BB is N 45° 00′ W (Fig(b)).

TAKE A EXAMPLE:

Find the back bearings of the following lines from the observed Fore Bearings:
1.PQ=79°24′ (WCB
2.RS = N28°12’E (QB)

SOLUTION:

1) The Fore bearing of PQ =79° 24′
Back bearing of PQ = FB + 180° (as the FB of PQ is less than 180°)
= 79° 24′ + 180° 00′
= 259°24′
(2) The Fore bearing of RS = N 28° 12′ E
Back bearing of RS = S 28 °12′ W