A compass is one of the Ten Essentials, a crucial piece of equipment you should carry on any outdoor adventure. Compasses vary widely in features and cost, however, from minimalist $10 models to high-end versions that can approach $100 or more. To help you navigate the options—and select the compass that’s right for you—consider these pointers on design, features, and functions.
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Basic compass types
A base plate compass features a free-spinning magnetized needle inside a rotating dial, or bezel, that is mounted to a flat piece of hard plastic (the base plate). It is the most useful—and recommended—design for backcountry navigation, especially for plotting directions to or from a map. A digital compass is often incorporated into GPS units, altimeter watches, and other handheld and wristtop devices. Though useful for general orientation, digital compasses aren’t designed for hands-on map work and depend on battery power to function. Coin-sized accessory compasses can be found on keychains, watches, and other tiny items; they are useful for rough directions but not much more.
|DID YOU KNOW?|
|The magnetic north pole shifts in response to small changes in motion inside Earth’s molten outer core. It has migrated more than 650 miles over the past century and is currently moving north-northwest at a rate of 25 miles per year.|
A rough orientation
Any compass, even the most inexpensive, will indicate the general direction north when held level and away from large metal objects and power lines (which interfere with the local magnetic field). In many cases, just this rough approximation will provide the information you need to make appropriate navigation decisions. Keep in mind, however, that compasses do not point due north. They point instead to the magnetic north pole, which is currently located in northern Canada. The difference between “true” north and “magnetic” north—known as the magnetic declination or variation—varies anywhere from zero degrees to 30 degrees or more, depending on your location. (In the Northeast, declination ranges from approximately 10 to 20 degrees.)
In order to identify or precisely navigate toward a specific landmark, you’ll want a compass that can provide a highly accurate bearing (the compass direction, usually measured in degrees, from one point to another). Look for large-diameter dials that feature small measurement increments; a mark for every two degrees or less is a good choice. Also consider a sighting mirror, which tilts upward from the base plate and enables you to see the landmark and compass reading simultaneously for maximum accuracy. Sighting mirrors fold down over the compass when not in use and protect it from wear and tear—a nice feature. They can also perform other vital tasks, such as signaling a passing aircraft in an emergency or evaluating the backcountry grunge on your face.
For map work, make sure the compass includes a series of parallel lines inside the dial. Known as meridian lines (or north-south lines), they are essential for plotting a field-obtained bearing onto the map or for measuring a bearing between two points on the map. (Some inexpensive models lack this important feature and should be avoided.) If you expect to plot extensive bearings, look for a base plate with a longer edge to better connect distant points on the map. Most compasses also include rulers along the edge of the base plate for quick measurement of distance; several usually match the standard scales of USGS topographic maps. These can be helpful, though an all-purpose inch or millimeter ruler is generally more useful.
Dial it in
For accurate navigation, you’ll need to account for magnetic declination. This is easier with a compass that indicates declination adjustment; look for it just inside the edge of the bezel. More expensive compasses allow you to turn the base plate manually inside the dial to account for a specific declination, usually by means of a tiny screw. Once properly set, such compasses conveniently provide true bearings, rather than magnetic bearings that require conversion. Other less important features to consider include a glow-in-the-dark needle and dial for nighttime navigation, an inclinometer for measuring slope angle in avalanche terrain, or a small base plate magnifying glass for up-close map evaluation.