Before GPS became ubiquitous, the compass was the primary navigation tool for wilderness travelers — and it remains the most reliable, never-running-low, waterproof piece of navigation gear you can carry. A quality baseplate compass and a USGS topographic map will get you home when electronics fail. Learning to use them together is a skill that pays dividends every time you step onto a trail.
How a Compass Works
A compass works because a magnetized needle is attracted to the Earth's magnetic north pole. The needle aligns itself with the horizontal component of the Earth's magnetic field, pointing roughly toward magnetic north. The difference between magnetic north and true north (geographic north) is called declination, and it varies by location — and changes slowly over time as the magnetic pole wanders.
The compass housing contains a rotating bezel marked with 360 degrees. North is 0° (or 360°), East is 90°, South is 180°, and West is 270°. Most baseplate compasses also have orienting arrows and a declination adjustment. Understanding these components is essential before you can take accurate bearings.
Taking a Bearing from the Field
To take a bearing to a distant landmark, hold the compass flat in front of you and turn your body until the direction-of-travel arrow points directly at the object. Then rotate the bezel until the orienting arrows inside the housing align with the north end of the magnetic needle — the needle will sit in the top of the orienting arrow. The number shown at the index line is your bearing: the direction to that object expressed in degrees.
Write the bearing down immediately. Bearings are easy to forget, especially if you're moving around. In your notebook, record bearing, object description, time, and any notable terrain features. This log becomes invaluable if you need to backtrack or share your location with rescuers.
Following a Bearing
To follow a bearing, set the desired degree number at the index line, then turn your body until the needle sits inside the orienting arrow. Your direction-of-travel arrow now points toward your destination. Walk toward it, periodically checking that the needle hasn't drifted from the orienting arrow — if it has, reorient your body. This is called "boxing the needle" and is the fundamental technique for staying on a bearing.
In practice, you won't walk a perfectly straight line. Terrain obstacles force detours. The key is to note your original bearing, deviate around the obstacle, then re-establish the same bearing on the other side. Don't try to compensate while circumventing — just return to your bearing afterward. For short obstacles like a fallen tree, walk at 90° to your bearing until you clear it, then turn back.
Map and Compass Together
Orienting a map to the ground is the first step in combining map and compass work. Place your compass on the map with the edge along a known north-south grid line. Then rotate the map and compass together until the compass needle aligns with the map's north — this puts the map in the same orientation as the landscape around you. You can now identify terrain features by comparing what you see to the map.
To plot a course from your map, you need to account for declination. If your local declination is 10° east, magnetic north is 10° west of true north — you need to subtract 10° from your map bearing (true bearing) to get your magnetic bearing, or add 10° to your magnetic bearing to plot on the map. Many compasses have an adjustable declination mechanism; set it once for your area and all bearings you take will already be corrected.
Triangulation
When you're uncertain of your exact position, triangulation uses two or more known landmarks to fix your location. Take a bearing to the first landmark and plot it on your map by drawing a line from that landmark along the bearing direction. Do the same with a second landmark. Where the two lines cross is your approximate position. A third bearing provides a "fix" and helps eliminate error.
For triangulation to work, you need to be able to identify landmarks on both the ground and the map. Choose prominent, isolated features — peaks, towers, river bends, or distinctive ridgelines. The more acute the angle between your two bearing lines, the more accurate the fix. Lines that intersect at roughly 90° are ideal; lines close to parallel give a wide margin of error.
Common Compass Errors
The most common mistake is letting metallic objects influence the needle — knives, watches, zippers, and even mineralized rock can deflect a compass needle by 10-20°. Always take compass readings away from metal objects. Another error is holding the compass at an angle rather than level — the needle must be free to swing horizontally to give an accurate reading.
Declination errors account for most navigation failures with compass and map. In areas with high declination (Alaska, northern Canada), failing to adjust for declination can put you walking in the completely wrong direction. Build the habit of checking your declination before every trip and adjusting your compass before you leave the trailhead.
Related Articles
- Navigation Without Compass — Stars, sun, and natural signs
- Map Reading Navigation — Topographic maps and terrain association
- Compass Bearing Tool — Practice taking and following bearings