Five Causes of Inaccurate Compass Readings
1. An inaccurate compass
Not all compasses are created equal. In fact, the difference between more basic models and top-end units is just about as vast as those between any other outdoor gear items, whether it be boots, backpacks, GPS devices, or clothing.
The most significant difference between compasses of differing quality lies in the potential for cross-range error when plotting lines of bearing.
Every compass has small bearing tick marks that are spaced at varying increments, usually ranging from 1° up to as much 5°. As you might imagine, the smaller the increments, the more accurate readings you’ll be able to take. Standard baseplate hiking compasses typically have increments of 2 or 3 degrees—a level of vagueness not conducive to precise navigation.
When buying your next compass, make sure its bearing marks are spaced in increments of no more than 2°, which is perfectly adequate for general hiking. If you’re an all-weather warrior and anticipate hiking in testy terrain in low visibility, then a model with 1° increments is the way to go.
2. Magnetic interference
Several magnetic forces can cause your compass to give an inaccurate bearing. The most common culprits are car speakers, smartphones, magnets on cellphone cases, avalanche beacons, GPS devices, cameras, car keys, ice axes, and even under-wired bras.
Keep your compass well away from magnets and metal objects at all times and hold the unit away from your body when using it. After taking a reading, cross-check features in the terrain with those on your map to ensure your compass bearing is correct.
If the polarity in your compass is reversed, you can fix the problem with a strong magnet by stroking the magnet outwards along the ‘north’ end of the needle to reset its magnetization.
3. Failing to adjust for magnetic declination
Magnetic declination refers to the variation in angle between magnetic north (that given by the magnetized needle and which points in the direction of north in the Earth’s magnetic field lines) and true north (the direction of the geographic North Pole).
Magnetic declination varies depending on your location and must be adjusted for, per the declination in your area, every time you take a bearing.
Buy a compass with a mechanism that allows you to adjust for magnetic declination or be prepared—and remind yourself!—to adjust or account for magnetic declination every time you take a bearing.
4. 180° out
A surprisingly common compass mistake is to align the wrong end of the compass needle, thus giving a reading that’s out to the tune of 180 degrees.
While this may seem like a Darwin-award-worthy error to make, in the heat of battle (a storm, for instance), you’d be far from the first to do so.
Take your time when taking readings and do a sense check of the reading by comparing the features in the nearby terrain with those on your map. Also, of course, bear in mind that the colored end of the compass needle is the magnetized end which points towards north.
5. Baseplate vagueness
Standard baseplate compasses are the type most commonly used by hikers and backpackers. Compared to the lensatic or prismatic compasses used by the military, baseplate models lack precision because they have no sighting system and are held at waist level when sighting an object in the terrain.
If you don’t want to go full-on prismatic or lensatic, choose a mirrored sighting compass. The mirrored black box component of these compasses has a small notch that allows the compass to sight on a distant feature far more precisely than when the compass is held at waist level.