Back Bearings (Reverse Bearings)
Grade 5Flipping the direction
If you know the bearing from A to B, you can work out the back bearing — the bearing from B back to A. This comes up in navigation problems and multi-stage journey questions.
The rule: if the bearing from A to B is less than 180°, add 180° to get the back bearing. If it is 180° or more, subtract 180°.
The reason this works is that the two North lines are parallel, so the back bearing differs by exactly 180° (co-interior angles with parallel lines).
Worked examples
EXAMPLE
The bearing from A to B is 065°. Find the bearing from B to A.
065° is less than 180°, so add 180°: 065° + 180° = 245°.
EXAMPLE
The bearing from P to Q is 310°. Find the bearing from Q to P.
310° is greater than 180°, so subtract 180°: 310° − 180° = 130°.
Always check three digits
After calculating, check the answer is still a three-figure bearing. A result like 65° needs to be written 065°.
