Understanding your Compass
Compasses come in several designs, but the best models have a rectangular base, free-moving north-seeking arrow, and rotating bezel ring. The floating compass card or arrow should be as detailed as possible rather than just showing the cardinal (north, south, east, west) and intercardinal (northeast, southeast, southwest, northwest) directions. The more detailed the compass, the more accurate your navigation will be if you pay attention to the rules.
It is also important to be familiar with other details of your compass, such as the increments used on the bezel ring. One compass may have a small line on the bezel ring for each degree, while another may have small lines that equal three degrees each. Over long distances, a mistake of two degrees will put you far off the mark.
Your compass may also have a mirror split with a sighting line for additional ease of bearing alignment and sighting. Some expensive models allow you to hold the compass to your eye and read the bearing via the mirror; others just add the mirror as a signaling device. The rectangular base of the compass should have a scale along its sides with inches, centimeters, and millimeters, used to measure distances on your map. Whatever compass you buy should be rugged and shock resistant, easily surviving bumps against rocks and pack frames.
The north-seeking arrow on your compass does not point to the North Pole or to grid north (which refers to the military's system of superimposed grid lines on a topographic map). Instead, your compass arrow points to a place on the earth where the planet's magnetic lines of force converge - magnetic north. Contrary to myth, there is no huge iron ore deposit or other anomaly there. Currently, magnetic north is in northern Canada; its location varies slightly from year to year.
Taking Bearings with Your Compass
With your feet about shoulder width apart and facing your objective, hold your compass at about chest level with index fingers along the sides of the base so that it is level and the north-seeking arrow floats freely (compasses that are not liquid-filled may have needles or compass cards that stick against the housing). Allow the needle to settle down and turn the rotating bezel ring until the alignment arrow painted onto the bottom of the compass card is directly below the north-seeking arrow. Now look at the top of the compass base and you will see an index or reference line (usually luminous). Read the bearing or azimuth on the bezel ring just below this line. That is the direction you are facing in degrees magnetic.
Mistakes To Avoid
So, forget about grid north altogether and concentrate on magnetic north and true (polar) north. Now we're down to two norths: magnetic north being that point in northern Canada where those mysterious magnetic lines of force come together, and true north, which is in the direction of the North Pole. The difference between the two, measured in degrees, is called declination or deviation. It varies depending on where you are, so check the declination diagram on your map to see what it is for the area you are in. If you are standing at a point where there is no declination, you are on an imaginary agonic line, or line of no variation. This means that there is no declination anywhere along this imaginary line.
We know the north-seeking arrow on your compass points to northern Canada, but it can also point to local magnetic disturbances called anomalies. Your camp stove might be one, as might your rifle, snowmobile, car, wristwatch, sunglass frames, radio, knife, or nearby power lines. It could also be a natural anomaly such as a subterranean ore deposit (magnetite). If you suspect that your compass is trying to fool you, check to see if something on your person or nearby might be pulling that north-seeking arrow off line. Don't know if a suspicious object is the culprit? Move the compass slowly toward the object and watch the arrow to see if it suddenly spins toward that object. If it does, you have probably found the problem, but remember that there can always be more than one anomaly. Nothing there? Perhaps you are near an ore deposit after all, so just move a couple of hundred yards away (if that is feasible) and try again.
Another common problem when using the compass is inadvertently following the north-seeking arrow. You shouldn't, unless you are heading directly magnetic north. Remember to sight along the sighting line or index line after lining up the north-seeking arrow and alignment arrow.
If your compass is a compensator model, which has a small screw on the bezel ring that allows the user to permanently adjust the compass for the declination in a particular region, be sure to change the setting when traveling in a different area. Otherwise, depending on the difference in declination between the two regions, you could be way off. Way off.
A compass of lesser quality may have a north-seeking arrow that jams against the inside of the bezel ring when it is not perfectly level. Avoid this problem by not buying a cheap compass. And if your compass has a slope scale, do not confuse it with the bezel ring's degree scale. A slope scale is a feature that allows you to judge the degree of slope of a distant hillside or mountainside.