| What it is | Angle between magnetic & true north |
| Varies by | Location and (slowly) time |
| Why it matters | Uncorrected error grows with distance |
| Fix | Adjust compass or correct bearings |
Declination (magnetic declination) is the angle between magnetic north, where a compass needle points, and true north, used by maps. It varies by location and slowly over time, and ignoring it can put you significantly off course over distance. You correct for it by adjusting your compass or adding/subtracting the local declination from your bearings.
Correcting for it
Set it on a baseplate compass so every bearing aligns with true north. Current values come from NOAA or your map.
Frequently asked questions
What is magnetic declination?
Declination is the angular difference between magnetic north (where your compass needle points) and true north (the direction maps are oriented to). It's expressed in degrees east or west and varies depending on where you are on Earth, because the magnetic poles don't coincide with the geographic poles.
Why does declination matter for navigation?
Because if you follow compass bearings without correcting for declination, your direction will be off by the declination amount — a few degrees that compound into hundreds of yards of error over a long leg. In areas with large declination, ignoring it can lead you well off route.
How do you adjust for declination?
The easiest way is to set the declination on a compass that has an adjustment, so all bearings are corrected automatically. Otherwise, you manually add or subtract the local declination (printed on the map or available from NOAA) when converting between map and compass bearings. Remember declination changes over time, so check current values.
Sources
- Magnetic declination — NOAA
- Compass declination — The Mountaineers