Bearings on the map
The compass is used as a protractor to both measure and plot bearings on a map. Magnetic north and magnetic declination have nothing to do with these calculations. Therefore, ignore the magnetic needle. Never make any use of the magnetic needle when taking or plotting bearings on a map. (The only time the magnetic needle is used on the map is whenever you choose to orient the map to true north, which was explained earlier in this chapter. But there's no need to orient the map simply in order to measure or plot bearings.)
To take (measure) a bearing on the map: Place the compass on the map with one long edge of the base plate running directly between two points of interest. As you measure the bearing from
Sighting notch mirror arrow
Fig. 4-5. Features of mountaineering compasses: a, essential features; b, useful optional features.
Sighting notch mirror arrow
Point A to Point B, see that the direction-of-travel line is pointing in the same direction as from A to B. Then turn the rotating housing until its set of meridian lines is parallel to the north-south lines on the map. (Be sure the orienting arrow that turns with the meridian lines is pointing to the top of the map, to north. If you point it toward the bottom, your reading will be 180 degrees off.)
Now read the number that is at the index line. This is the bearing from Point A to Point B.
In the example shown in figure 4-6, the bearing from Point A, Panic Peak, to Point B, Deception Dome, is 40 degrees.
If your map doesn't happen to have north-south lines, just draw some in, parallel to the edge of the map and at intervals of an inch or two.
To plot (follow) a bearing on the map: In this case you are starting with a known bearing. And where does that bearing come from? From an actual landscape compass reading. Let's take a hypothetical example (fig. 4-7): A friend returns from a trip, disgusted at himself for leaving his camera somewhere along the trail. During a rest stop, he had taken some pictures of Mount Magnificent. At the same time, he had taken a bearing on Mount Magnificent and found it to be 135 degrees. That's all you need to know. You're heading into that same area next week, so get out the Magnificent
quadrangle, and here is what you do:
First set the bearing of 135 degrees at the compass index line. Place the compass on the map, one long edge of the base plate touching the summit of Mount Magnificent, with the direction-of-travel line pointing toward Mount Magnificent. Rotate the entire compass (not just the housing) until the meridian lines are parallel with the map's north-south lines, and make sure the edge of the base plate is still touching the summit. Again, be sure the orienting arrow points to the top of the map, toward north. Follow the line made by the edge of the base plate, heading in the opposite direction from the direction-of-travel line because the original bearing was measured toward the mountain. Where the line crosses the trail is exactly where your friend's camera is (or was).
Continue reading here: Bearings in the field
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