As you've gathered by now, there's a big difference between using a compass for working with a map and using a compass for field work. When measuring and plotting bearings on a map, the compass needle is ignored. Just align the meridian lines on the compass housing with the north-south lines on the map. In the field, you must use the magnetic needle.

You may have heard that metal can mess up a compass reading. It's true. Ferrous objects—iron, steel, and other materials with magnetic properties—will deflect the magnetic needle and produce false readings, as will a battery-powered watch that is within 6 inches of a compass. Keep the compass away from belt buckles, ice axes, and other metal objects. If a compass reading doesn't seem to make sense, see if it's being sabotaged by nearby metal.

Keep your wits about you when pointing the declination arrow and the direction-of-travel line. If either is pointed backward—an easy thing to do—the reading will be 180 degrees off. If the bearing is north, the compass will say it's south. Remember that the north-seeking end of the magnetic needle must be aligned with the pointed end of the declination arrow. And that the direction-of-travel line must point from you to the objective, not the reverse.

There's yet another way to introduce a 180-de-grcc error in a compass reading. The way to do it is to align the compass meridian lines with the north-south lines on a map, but have the declination arrow pointing backward. The way to avoid this is to check that your declination arrow is pointing more or less to north (rather than more or less to south). This check has nothing to do with declination. It just happens that the arrow is placed in a convenient spot to serve as a reminder of which way to direct the meridian lines.

If in doubt, trust your compass. The compass, correctly used, is almost always right, while your contrary judgment may be clouded by fatigue, confusion, or hurry. If you get a nonsensical reading, check to see you aren't making one of those 180-

degree errors. If not, and if there is no metal in sight, verify the reading with other members of the party. If they get the same answer, trust the compass over hunches, blind guesses, and intuition.

Continue reading here: How Altimeters Aid Mountaineers