Using back bearings
If you are not sure where you are, you can calculate your position on the map by using known points of reference. For example, if you are walking along a lengthy featureless ridge but are not sure how far you have walked, you can use any identifiable features to either side or ahead of the ridge which are also on your map to help you find your exact position on that ridge. Likewise if you are uncertain where you are on the map, yet you can identify two, or preferably three features that are also on the map, you can calculate your position by resection or triangulation.
To perform a resection or triangulation:
- Identify a feature which is shown on the map you have.
- Take magnetic bearings from your position to that feature.
- Convert that bearing to a back bearing (i.e. either add or subtract 180°).
- Convert the magnetic back bearing to a grid bearing.
- Now plot that bearing on the map by drawing a line from the known feature you have selected in the direction of the bearing you have calculated.
- Repeat this procedure with two other known features on the map.
In theory the result will be a point where the three lines intersect. In practice the result is often a small triangle where the three lines meet. Your position lies somewhere within that triangle. Using back bearings to perform a resection or a triangulation always takes some time, even when you are proficient. It is worthwhile practising before you really need to use these techniques!