How to apply it
- 1Find the declination for your area on the map's declination diagram (or a current source — declination drifts over years).
- 2Note which way it points — magnetic north east or west of true north — and by how many degrees.
- 3Convert your bearing with math — apply the declination to go between the map bearing and the magnetic bearing you follow on the compass.
- 4Watch grid vs. true vs. magnetic — if you took the bearing off the map's grid lines, correct grid→true first, then true→magnetic.
- 5Don't eyeball the diagram — declination diagrams aren't drawn to scale; use the printed number and arithmetic.
- 6Re-check on long trips — a few degrees compounds; confirm your position against terrain as you go.
When to use it
- Any accurate map-and-compass navigation over meaningful distance.
- Orienteering and backcountry travel, where a few degrees at the start becomes hundreds of yards at the finish.
- Understanding why "the compass doesn't point to the North Pole" — and why that's fine once you adjust.
Common mistakes & safety
- Ignoring declination is a slow-motion error. Small on a short leg, but over miles a several-degree offset lands you well away from your target.
- Use the number and math, not the picture. Declination diagrams are schematic, not to scale — measuring an angle off the diagram gives a wrong correction.
- Know east vs. west declination. Applying the correction the wrong way doubles the error instead of removing it — get the direction of adjustment right.
- Declination changes over time and place. An old map's value can be off by degrees now — check a current source for the exact figure.
- Terrain-check your progress. Even correct math can't fix a misread bearing — confirm against landmarks.
Going deeper
- Three norths. True north (the map's geographic north), magnetic north (where the needle points), and grid north (the direction of the map's grid lines) — all slightly different, and the declination diagram shows how.
- "East is least, west is best" is a common memory aid for which way to apply declination — but the reliable method is to reason it out with the diagram and math, not a rhyme.
- Declination drifts. The magnetic pole moves, so a map's stated declination slowly goes out of date — for precise work, look up the current value for your location and date.
Official source
How to use a map and compass — Scout Life →Every topographic map prints a declination diagram. Declination appears in the Scouts BSA navigation requirements and the Orienteering merit badge.