Magnetic Declination Explained
Understanding the Difference Between True North and Magnetic North
Magnetic declination is the angle difference between true north and magnetic north. Understanding this concept is essential for accurate navigation with any compass. Learn how declination affects your compass readings and how to compensate for it.
What is Magnetic Declination?
When you use a compass, the needle points toward magnetic north, not the geographic North Pole (true north). The angle between these two points is called magnetic declination or magnetic variation.
This difference occurs because Earth's magnetic field is generated by molten iron in the planet's core, and the magnetic poles don't align with the geographic poles. The magnetic north pole is currently located in northern Canada, about 500 km from the true North Pole.
East vs West Declination
- East Declination (positive): Magnetic north is EAST of true north. Common in western North America and western Europe.
- West Declination (negative): Magnetic north is WEST of true north. Common in eastern North America and eastern Asia.
- Zero Declination (Agonic Line): Magnetic and true north align. Runs roughly through the central United States.
Magnetic Declination by Region
| Location | Approximate Declination |
|---|---|
| New York, USA | -13° (West) |
| Los Angeles, USA | +12° (East) |
| London, UK | -1° (West) |
| Sydney, Australia | +12° (East) |
| Tokyo, Japan | -7° (West) |
| Berlin, Germany | +3° (East) |
| São Paulo, Brazil | -21° (West) |
| Mumbai, India | -1° (West) |
Note: Declination values change over time. Always verify current values for precise navigation.
How to Adjust for Magnetic Declination
The Memory Aid: "East is Least, West is Best"
When converting from magnetic to true north:
- East declination: SUBTRACT the declination from your magnetic reading
- West declination: ADD the declination to your magnetic reading
Example Calculation
Your compass reads 45° magnetic, and your local declination is 10° East:
True Bearing = 45° - 10° = 35° True North
Why Does Declination Change?
Magnetic declination is not constant. It changes due to:
- Location: Declination varies dramatically across the globe
- Time: The magnetic poles drift about 55 km per year
- Solar activity: Magnetic storms can cause temporary variations
- Local anomalies: Iron deposits can create local magnetic disturbances
Finding Your Local Declination
Several resources provide current magnetic declination:
- NOAA Calculator: ngdc.noaa.gov/geomag/calculators/magcalc.shtml
- Topographic Maps: Display declination in the legend
- Hiking GPS Units: Many have built-in declination databases
- Smartphone Apps: Some compass apps auto-adjust for declination
When Does Declination Matter?
- Wilderness Navigation: Critical for hiking and orienteering
- Aviation: Pilots must account for variation on charts
- Maritime: Ships use corrected compass courses
- Surveying: Land surveys require true north bearings
For casual use like finding general direction, declination adjustment is often unnecessary.