In PART 9 of our series on navigation techniques, Lyle Brotherton explains how back bearings can be used to stop you getting lost.
"If you’re unsure of your exact position and you don’t have the luxury of GPS it’s possible to work out where you are the old-fashioned way with a map and compass. You can also use these tools to check your navigation as you go, and one of the easiest techniques for this is using back bearings."
Back bearings can be employed quite simply to check that you have walked on the correct bearing (called ‘leg confirmation’). Plus, with a bit of practice, they can also be utilised to pinpoint your location more precisely using just your map and compass (‘resection’). The easiest way to think about back bearings is this: whatever bearing the red north needle of your compass is pointing to, the white south needle (also sometimes black) will be pointing in the opposite direction – this is your back bearing (see diagram below).
Using back bearings to check navigation (leg confirmation)
To follow your route to your destination you navigate a series of short legs. Each leg starts from a known point and leads to an identifiable point on the map (known as an attack point).
When you arrive at your attack point, turn around and point your compass in the direction of the point you have just walked from. If the white needle of the compass aligns exactly where the red needle of the compass was you are on the right track!
Sometimes an attack point may not be a particularly obvious feature, so it's important to confirm that you have accurately followed the last leg.
- Once you believe you have reached your planned attack point, without changing the bearing hold the compass squarely out in front of you at about waist height. Lean slightly over to look down on it and let the compass needle float freely. It will point to magnetic north.
- Rotate your whole body (not just the compass) until the white (or black) end of the compass needle is exactly aligned with the orientating arrow in the bottom of the compass bezel housing (see images above). This is your back bearing (180° to your original bearing).
- If you’ve followed your original bearing accurately your compass’s direction-of-travel arrow should be pointing at the place you have just come from.
Using back bearings to confirm your position (resections)
This is the use of back bearings to establish where you are. It is particularly effective if you are on or near a linear feature such as a path, river or the edge of a wooded area.
- Standing by your linear feature, look for a prominent feature in the landscape (such as the summit of a hill) which will also be identifiable on your map.
- Turn and face this prominent feature straight on.
- Orientate your map by placing your compass on it and rotating both together until the red needle of your compass is pointing to the top of your map parallel to the north/ south grid lines.
- With the map orientated, draw an imaginary line from the prominent feature in the landscape through the same feature on your map and straight back to you. Where this imaginary line intersects, the linear feature on the map is a rough guide to your position.
Precise method (sometimes called the ‘cocked hat’ technique)
This is not a technique you will need to use very often, but it could be invaluable in vast featureless areas. If the three lines do not precisely intersect in the same point, you will end up with a small triangle where they meet. Your approximate position is somewhere within this triangle.
- Locate a prominent feature in the landscape (such as the summit of a hill) which will also be identifiable on your map.
- Take a back bearing to this feature and mark this as a straight line on your map.
- Repeat this process for another two prominent features.
- Where all three lines intersect on your map is an accurate estimation of your exact position.