Introduction to the Compass
This course uses the lensatic compass, the Cammenga type the Army and Marines carry. Everything here also works on a clear baseplate compass. The parts just have different names.
Layout of the compass
Tap each part.
The three norths
"North" means three different things. Your map shows all three in a small diagram in the margin.
The direction to the North Pole, the top of the globe. Lines of longitude point here.
Where your compass needle points. It's a spot in the Arctic that drifts every year.
The direction the grid lines on your map run. It's very close to true north, but not always exact.
The gap between grid north and magnetic north is what you correct for when you use a map and compass together. Step 3 covers it.
Northern vs. southern hemisphere
- True north is always toward the North Pole, anywhere on Earth. In the south, people often navigate by true south instead, because it's the closer pole.
- Night sky check: in the north, the North Star (Polaris) sits almost exactly over true north. In the south there's no pole star. Use the Southern Cross to find true south.
- Compass balance: Earth's magnetism pulls the needle down at an angle, and the angle changes by hemisphere. A compass balanced for North America can drag and stick in Australia or South America. Traveling far south? Buy a compass marked "global" or balanced for that zone.
Dog in the shed
The rule you'll use every single time: the dog goes in the shed. The dog is the north end of the needle or dial arrow. The shed is the orienting arrow or glowing bezel line. When the dog is in the shed, your compass is lined up and the index line or travel arrow points your way. Some instructors say "red in the shed." Same rule.
Orienting the Compass, Azimuths & Back Azimuths
An azimuth is a direction measured in degrees, clockwise from north. North is 0°, east 90°, south 180°, west 270°.
Orienting your compass to the map
- Lay the map flat. Place the compass on it with the straight edge along a north-south grid line and the cover pointing to the top of the map.
- Turn the map and compass together, as one piece. Don't move the compass on the map.
- Stop when the dial reads your correction. East declination: 360 minus the declination (at 15° east, stop at 345°). West declination: stop at the declination number.
- The map now points the same way as the ground. Roads, ridges, and streams on the map line up with the real ones.
Shooting an azimuth (no map)
- Open the cover straight up. Tilt the lens back about 45°.
- Thumb through the loop. Raise it to your eye and brace it on your cheek.
- Look through the rear slot and line the sighting wire up on your target.
- Glance down through the lens. The number under the index line is your azimuth.
Walking it fast: use the center-hold. Cover open flat, compass level at your belt buckle, elbows tight. Turn your whole body until your azimuth sits under the index line, then walk.
Practice: set your dial
Back azimuth: the way home
A back azimuth is the exact opposite direction. Use it to return to where you started.
- Azimuth under 180°: add 180.
- Azimuth 180° or more: subtract 180.
Setting an azimuth in the dark
You can't read numbers at night, so count bezel clicks. 1 click = 3°.
- Turn the bezel until the glowing line sits over the black index line.
- Divide your azimuth by 3. That's your click count.
- Turn the bezel counterclockwise that many clicks.
- Turn your body until the glowing north arrow sits under the glowing bezel line. Dog in the shed. Walk.
Understanding the Map
Marginal information: read the edges first
Before you look at the terrain, read the margins. They tell you how to read everything else.
Pace Count
Find your pace count
Your compass tells you which way. Your pace count tells you how far. Without it, you won't know when you've arrived or when you've gone past.
Understanding Terrain
5 major terrain features
Make a fist and look at your knuckles. The knuckles are hills, the low spots between them are saddles, the line across the knuckles is a ridge, and the grooves running down your fingers are draws. That's how instructors teach terrain without a map.
Reading the Map Grid Square
The rule: read right, then up
Military maps and newer USGS topos are covered in a grid of 1,000-meter squares. Search and rescue teams and FEMA use the same system, called the US National Grid. A grid coordinate tells someone exactly where you are.
Making the Map and Compass Work Together
From the map to the ground
Don't stare at the compass. Keep matching the ground to the map. "I should cross a stream, climb a spur, then hit a saddle." If the ground stops matching, stop and figure out why before you go farther.
Q&A and Final Check
Common questions
Batteries die, cold kills them faster, screens crack, and canyons and thick forest block signal. A compass has no battery. Use GPS as a backup check, not the plan.