Table of Contents
What Is Magnetic Declination?
Magnetic declination (or magnetic variation) is the angle between true north (geographic north pole) and magnetic north (where a compass needle points). This angle varies depending on your location on Earth because the magnetic poles do not coincide with the geographic poles. The north magnetic pole is currently located in the Canadian Arctic, about 500 km from the geographic north pole, and it moves about 40-50 km per year.
Understanding declination is critical for navigation with a magnetic compass. If you follow a compass heading without correcting for declination, your actual track will deviate from your intended direction. The error compounds with distance: a 10-degree declination error causes you to be 1.7 km off course for every 10 km traveled.
Calculation Method
This calculator uses a simplified dipole model of Earth's magnetic field. For precise values, use the World Magnetic Model (WMM) or International Geomagnetic Reference Field (IGRF), which account for the complex multipole structure of the real field. This estimate is typically accurate to within 2-5 degrees.
Declination by Region
| Location | Approx. Declination | Direction |
|---|---|---|
| New York, USA | -13° | West |
| London, UK | 0° | Agonic line |
| Sydney, Australia | +12° | East |
| Tokyo, Japan | -7° | West |
| Sao Paulo, Brazil | -22° | West |
Frequently Asked Questions
What is the agonic line?
The agonic line is an imaginary line on Earth where magnetic declination is zero - where true north and magnetic north coincide. Currently, it runs roughly from the Great Lakes through Florida and into South America. East of this line (in the US), declination is west; west of it, declination is east.
Does declination change over time?
Yes, significantly. The magnetic poles wander continuously due to changes in Earth's liquid iron core convection. The north magnetic pole has moved about 1,100 km since 1831 when it was first located. Current movement is about 55 km/year toward Siberia. Topographic maps print the declination at time of publication, which may be years out of date.
How do I correct my compass for declination?
For west declination: add the declination to your magnetic reading to get true bearing. For east declination: subtract. Many quality compasses have an adjustable declination ring that lets you set the correction once so all readings are automatically in true bearing.