Key takeaways
- Declination is the angle between true north (map) and magnetic north (compass needle).
- It varies by geographic location and changes slowly over years.
- You must adjust for declination to convert accurately between map and compass bearings.
- Many compasses let you set a declination correction; otherwise add/subtract it manually.
What declination is
Declination — magnetic declination — is the angle between true north and magnetic north. True north is the direction to the geographic North Pole, which maps are drawn to. Magnetic north is where your compass needle actually points. The two rarely line up, and the angle between them is the declination you must account for.
Why it matters
Because maps use true north but your compass uses magnetic north, failing to correct for declination throws your bearings off. And a small angular error compounds over distance — a few degrees can put you hundreds of meters off course on a long leg.
Before a trip, a navigator looks up the area’s current declination (say 14° E), sets that correction on their adjustable compass, and from then on every bearing they take or follow automatically reconciles map and compass — no mental math on the move.
How to adjust for it
The simplest method is a compass with an adjustable declination feature: set your local value once and all readings are corrected automatically. Otherwise, add or subtract the declination manually when converting between map and compass bearings. Find the current value on an up-to-date map or from NOAA — it drifts slowly over the years, so use a recent figure.
The bottom line
Declination is the gap between the map's true north and your compass's magnetic north — and ignoring it quietly wrecks your navigation, since small angular errors compound over distance. Look up your area's current declination (it drifts over years), set it on an adjustable compass or correct manually, and your map and compass will finally agree.
Frequently asked questions
What is magnetic declination?
Magnetic declination is the angle between true north (the direction to the geographic North Pole, used by maps) and magnetic north (the direction your compass needle points). Because these two norths usually differ, the angle between them — the declination — must be accounted for to navigate accurately with a map and compass.
Why does declination matter for navigation?
Maps are drawn to true north, but compasses point to magnetic north, so if you don't correct for the difference your bearings will be off — and even a few degrees of error compounds into being far off course over distance. Adjusting for declination keeps your map and compass work consistent and accurate.
How do you adjust for declination?
The easiest way is to use a compass with an adjustable declination feature, set to your area's current declination so all readings are automatically corrected. Otherwise, you add or subtract the declination manually when converting between map (true) and compass (magnetic) bearings. Find your local value on a current map or from NOAA, since it changes over time.
Sources
- Magnetic declination & geomagnetism — NOAA
- Compass navigation — The Mountaineers