What Is Magnetic Declination & How to Calculate It for Accurate Navigation

Because the Earth's geographic and magnetic poles are located hundreds of miles apart, a compass needle will rarely point along the true geographic meridian. The horizontal angle between True North and the direction your compass needle actually points is called Magnetic Declination (sometimes termed magnetic variation by pilots and mariners).

Depending on where you are located on the globe:

• Easterly Declination: Magnetic North sits to the East of True North. Declination is recorded as an angle like +12° E.
• Westerly Declination: Magnetic North sits to the West of True North. Declination is recorded as -14° W.
• Agonic Line: The line across Earth where declination is exactly 0°. Compasses located along this line point directly at both True and Magnetic North simultaneously (running through parts of Lake Michigan and Florida).

To transfer a bearing measured on a paper map with a protractor into a heading you can follow with a magnetic compass, use the standard navigation equation:

Over centuries of marine and wilderness navigation, instructors developed simple rhymes to remember whether to add or subtract when converting from Map (True) to Compass (Magnetic):

Because Earth's liquid core churns constantly, declination values are not static numbers. If you look at the bottom margin of an older USGS topographic map printed in 1985, you might read: "Approximate mean declination 1985: 12°W (Annual change: decrease 0.1°)".

Over 40 years, that 0.1° annual shift accumulates to a 4-degree difference! In aviation and professional forestry, using outdated declination figures can result in landing runway errors or misplacing property boundaries. Always verify current values using NOAA's digital geomagnetic calculator.