A compass tells you where magnetic north is. A map is drawn to true north. The angle between them — magnetic declination — changes by location and year. Ignoring declination produces navigation errors up to 1/4 mile per mile traveled in areas with high declination. This guide covers the full declination correction procedure and the step-by-step bearing technique that eliminates this error. The context for using compass with a topographic map is in land navigation without GPS: map, compass, and terrain reading.
Magnetic North vs True North
True north is the geographic North Pole — the axis of Earth’s rotation. Topographic maps are aligned to true north. Magnetic north is the location of Earth’s magnetic field concentration in the Canadian Arctic. A compass needle points to magnetic north, not true north.
The angle between true north and magnetic north at any given location is the magnetic declination. This angle changes by geographic location and changes slightly each year as Earth’s magnetic field shifts. Current declination for any U.S. location is available at ngdc.noaa.gov/geomag/calculators/magcalc.shtml.
Declination by region (approximate 2024 values):
- Pacific Northwest (Seattle, Portland): 15–18° East — compass points 15–18° to the right of true north
- Rocky Mountains (Denver, Salt Lake City): 8–11° East
- Midwest (Chicago, Minneapolis): 2–4° West
- Southeast (Atlanta, Nashville): 5–8° West
- New England (Boston, Portland ME): 14–16° West
Declination Correction: The Rule
There are two conversions you need: true bearing → magnetic bearing (when you’ve plotted a bearing on a map and need to follow it in the field), and magnetic bearing → true bearing (when you’ve taken a field bearing to a landmark and need to plot it on the map).
The memory rule: “East is least, West is best.”
- East declination: Subtract declination from true bearing to get magnetic bearing (for field use). Add declination to magnetic bearing to get true bearing (for map plotting). True = Magnetic + Declination.
- West declination: Add declination to true bearing to get magnetic bearing (for field use). Subtract declination from magnetic bearing to get true bearing (for map plotting). True = Magnetic − Declination.
Example: You plot a bearing of 045° true on a map (northeast). Your location has 15° East declination. Magnetic bearing for field use: 045° − 15° = 030° magnetic. You set 030° on your compass bezel and follow it in the field.
Adjustable declination compasses: The Silva Ranger and Suunto A-2000 allow you to set the declination offset permanently on the compass, so the orienting arrow is pre-rotated to account for declination. This eliminates the mental arithmetic in the field. Check the compass manual for the adjustment procedure — it takes about 2 minutes.
Taking a Field Bearing: Full Procedure
- Step 1. Face the landmark you want to take a bearing to.
- Step 2. Hold the compass level at waist height, with the direction-of-travel arrow pointing directly at the landmark.
- Step 3. Rotate the bezel until the orienting arrow aligns under the north end (red end) of the compass needle. Mnemonic: “put red Fred in the shed” — the red needle sits inside the red orienting arrow outline.
- Step 4. Read the bearing at the index line (the mark at the base of the direction-of-travel arrow on the compass body). This is your magnetic bearing to the landmark.
- Step 5. Apply declination correction to convert to true bearing if plotting on a map. If your compass is set to the local declination, no calculation needed.
Following a Set Bearing
- Set the desired bearing on the bezel (rotate bezel until bearing aligns with index line).
- Hold the compass level. Rotate your body until the red needle sits in the orienting arrow.
- The direction-of-travel arrow now points in your bearing direction. Look up and identify a specific landmark in that direction — a specific tree, rock formation, or terrain break.
- Walk to that landmark without looking at the compass. Re-check the bearing at the landmark and pick the next intermediate point. Repeat until you reach your destination or can identify your position on the map.
Critical discipline: Walk to intermediate landmarks, not with your eyes on the compass. Staring at a compass while walking causes gradual drift off-bearing as you unconsciously adjust your path. The intermediate landmark technique maintains ±2–5° bearing accuracy over terrain; constant-compass-watching produces ±10–15° accuracy over the same distance.
Back Azimuth
The back azimuth is the reverse bearing — the direction back to where you came from. Calculate it by adding or subtracting 180°:
- If your forward bearing is less than 180°, add 180°: forward bearing 045° → back azimuth 225°
- If your forward bearing is 180° or greater, subtract 180°: forward bearing 270° → back azimuth 090°
The back azimuth is the bearing you’d follow to return to your starting point — useful for navigation in unfamiliar terrain or during resection (two-bearing fix) to determine your position on the map.
Compass Errors to Avoid
- Metal interference: Keep the compass at least 12 inches from metal objects (rifle, knife, watch, belt buckle) when taking readings. Magnetic items (magnetized knife blades, rifle scope mounts) can deflect the needle by 5–20°.
- Tipping the compass: A level compass reads accurately. On a slope or tilted, the needle drags on the compass housing, producing friction errors. Keep it horizontal.
- Reading the wrong end: The red north end of the needle must align with the orienting arrow, not the white/black south end. Aligning the south end produces a bearing 180° off.
- Not applying declination: In high-declination areas (Pacific Northwest, New England), failing to correct for declination produces a 0.25–0.30 mile error per mile traveled.
Where to Go Next
Using these compass techniques with a topographic map — including resection, pace counting, and terrain association — is covered in land navigation without GPS: map, compass, and terrain reading. Reading topographic maps — contour patterns, drainage systems, and USGS symbol legend — is in topographic map reading: terrain features and navigation symbols.
