Magnetic variation

Magnetic variation is the angle between true north and magnetic north at a particular location. It changes with geographic position and slowly changes over time. Charts publish variation information so pilots can convert between true directions and magnetic directions used by many aircraft instruments and procedures.

How it works

True north points toward the geographic North Pole. Magnetic north is the direction indicated by a magnetic compass in a particular area. The difference between them is called variation. A chart may show an isogonic line, an agonic line, or a magnetic variation value for the charted area.

Variation is not the same as compass deviation. Variation is an earth reference effect. Deviation is an aircraft and compass error caused by magnetic fields within the aircraft. Pilots account for both when a procedure requires a magnetic heading.

Example

If a charted location has 10° east variation, a true course and magnetic course differ by 10 degrees. The pilot applies the convention and the aircraft's approved navigation procedure to convert the value. The correct sign depends on the convention being used, so the chart legend and flight planning method matter.

Why it matters

A small conversion error can become noticeable over a long route or when intercepting a narrow course. Magnetic variation also explains why runway numbers, true geographic directions, and navigation display values may not match exactly.

Use current charts and the aircraft's navigation references. The published variation may be tied to the chart's effective period, and local magnetic conditions can change over time. Do not rely on an old chart for current navigation data.

Related terms and tools

• Bearing
• True heading
• FAA Aeronautical Chart User's Guide