This is the classic map reading skill, where you can determine your location accurately by pinpointing two or three other features.
Transit Lines – This is when a number of features will line up directly from one location. This will provide you with a perfect line – and you’ll be located somewhere along that line. If you’re on a linear feature like a ridge or path, then you can pin point your location pretty accurately. The Bwlch used in the example below may not be the best feature, as it may not be as clear cut as other features. As it happens, the image was taken near the top of the Gribin ridge, which lines up perfectly with the arrow on the map.
Determining it with one landmark – Back Bearing – You can do this by using your compass, but you’ll need to be located along a known linear feature. Take a bearing on a known landmark (e.g. a summit), and then do a backbearing on your map. You’ll be somewhere along the line, and where it intersects your linear feature is your location.
For example, you’ve taken a bearing on the landmark as above and found that it’s located at a bearing of 40°. You then essentially need to transfer that bearing onto the map in order to determine what location is essentially in the opposite direction to your bearing – but from the landmark. You do this by lining up the orienting lines on the compass with the eastings on the map, and then move the compass until the edge of the baseplate lines up with the landmark. The baseplate should intersect the linear feature (path in this case) at your location.
This is essentially just the process of taking a bearing from the map and then following it on the ground rather than the usual way around. Try and get your bearing to intersect your linear feature as close to 90° as possible.
Determination with one landmark and an altimeter. You can likewise turn the contours on the mountain into a known linear feature. If the altimeter places you at 800m, then where the triangulation backbearing crosses the 800m contour is roughly where you are.
Resectioning – You’re not on a linear feature, and not really sure where you’ve found yourself! In this instance, if you can identify two or more landmarks, you can locate yourself by taking a backbearing towards both landmarks, or ideally three. The landmarks should be as far apart on the compass, rather than in the same direction as this makes it more accurate. Technically, you need to draw a line on with pencil. Repeat the process with a second and perhaps a third landmark, preferably ones in different directions. The lines should cross or enclose a small area, which is precisely where you are.
In this example we took the first backbearing from the minor summit (1) that was literally at 0°. We knew we were on a summit, so one could have been enough, but we decided to take another backbearing to point (2) and just for the hell of it, we took a final one for summit (3) as well! No doubt that we’re somewhere within the small triangle created by the intersecting lines. These three landmarks are also pretty far apart on the compass, which makes the result more accurate.
This is a skill you should practice, and you’ll use the backbearing to a linear feature much more often than any other. Resectioning isn’t particularly practical on a mountain, unless you’re planning on brining a pencil case on every trip!