If you’ve ever tried to navigate precisely with a map and compass and found your route drifting off course, magnetic declination may be the culprit. It’s the most frequently misunderstood concept in compass navigation — and one of the most important to get right.
What Is Magnetic Declination?
Magnetic declination (also called magnetic variation) is the angle between magnetic north — the direction your compass needle points — and true north — the direction toward the geographic North Pole.
The two are not the same, and the difference between them is declination. It is expressed in degrees east or west:
- East declination (positive): Magnetic north is east of true north. To find true north, turn slightly left from where your compass needle points.
- West declination (negative): Magnetic north is west of true north. To find true north, turn slightly right.
Declination values around the world currently range from about 25° east (parts of Siberia and Alaska) to about 30° west (parts of the Arctic). In well-populated areas, it typically falls between 0° and 20°.
Why Does Magnetic Declination Exist?
Earth behaves like a giant bar magnet with a north and south magnetic pole. However, the magnetic poles do not coincide with the geographic (rotational) poles. The magnetic north pole is currently located in the Canadian Arctic, at roughly latitude 86°N and longitude 160°W — many hundreds of kilometres from the geographic North Pole at 90°N.
This offset means that from most places on Earth, the direction toward the magnetic pole and the direction toward the geographic pole are measurably different.
Why Does It Change Over Time?
Unlike the geographic poles, which are essentially fixed, the magnetic poles wander continuously as convection currents in Earth’s liquid outer core shift the planet’s magnetic field. The magnetic north pole has moved roughly 1,000 km northward since the 1990s and is currently drifting toward Siberia at a rate of about 35–55 km per year — faster than at any point in recent history.
This means declination values change over time. A declination value measured ten years ago may be off by several degrees today. Navigation charts and apps update their declination figures regularly using the World Magnetic Model (WMM), which is recalculated every five years.
How Much Does Declination Matter?
This depends on the magnitude of your local declination and the distances involved.
A simple formula: for every degree of uncorrected declination, you will be off by about 17 metres per kilometre of travel. So if you walk 10 km on an uncorrected bearing with 10° of declination, you will end up about 1.7 km from your intended destination.
| Declination | Error per km | Error at 10 km |
|---|---|---|
| 1° | ~17 m | ~175 m |
| 5° | ~87 m | ~870 m |
| 10° | ~174 m | ~1.7 km |
| 20° | ~342 m | ~3.4 km |
For a short stroll in a park in central Europe (declination ~1°), the error is negligible. For a multi-day wilderness hike in Alaska (declination ~18°), ignoring it could leave you kilometres off course.
How to Find Your Local Declination
The most accurate way is to use a dedicated geomagnetic model. Our Star Compass calculates your declination from your GPS coordinates using a simplified dipole geomagnetic model — good enough for outdoor navigation purposes.
You can also find declination values from:
- NOAA’s online declination calculator (uses the official WMM model)
- Printed topographic maps — declination diagrams in the map margin show the local value at the map’s publication date (check how old the map is and adjust)
- GPS navigation apps — most apply declination automatically to give true north bearings
How to Apply Declination Correction
Memorising the Rule
The classic memory aid is “East is least, West is best”:
- If declination is East: the magnetic bearing is larger than the true bearing, so subtract the declination to get true north.
- If declination is West: the magnetic bearing is smaller than the true bearing, so add the declination to get true north.
Example
You’re hiking in Colorado where declination is approximately 9° East.
- Your compass reads 045° (northeast).
- Subtract the 9° east declination: 045° − 9° = 036°.
- Your true bearing is 036° (approximately north-northeast).
Adjustable Compasses
Many quality hiking compasses have an adjustable declination mechanism — you rotate a small ring to set your local declination, and from then on the compass automatically corrects every bearing. Check your compass manual.
Digital Tools
Our Star Compass and Online Compass tools display the magnetic heading from your device’s sensor directly. The Star Compass additionally shows your local declination and the direction to Polaris (true north), so you can see the difference in real time without any calculation.
Magnetic Declination vs. Grid Declination
One more layer of complexity: topographic maps use a grid north that aligns with the map’s vertical lines (the UTM grid), which differs slightly from both magnetic and geographic north in most places. When navigating with both a compass and a detailed topo map, you may encounter three-way diagrams showing the angles between true north, magnetic north, and grid north. For recreational hiking, this level of detail is rarely needed — the difference between grid and geographic north is usually under 1° — but it’s worth knowing the concept exists.
The Bottom Line
Magnetic declination is the angle between where your compass points (magnetic north) and where true north actually is. It varies by location, changes over time, and can cause significant navigation errors on long routes if left uncorrected. For casual navigation, check your local value and mentally adjust. For precise map work, use an adjustable compass or a tool that applies the correction automatically.