Variation and Deviation

Variation Explained

Variation (also called magnetic variation or declination) is the angle between true north (geographic north) and magnetic north at a specific location. This angle can be east or west, depending on whether magnetic north lies east or west of true north at that point. Variation values are shown on aeronautical charts, often with isogonals (lines of equal variation) and annual change rates, as the Earth's magnetic field slowly shifts over time. Pilots must apply variation to convert between true and magnetic headings or bearings.

How Variation Changes

Variation is not constant—it changes with your position on the Earth and gradually over time due to the movement of the magnetic poles. Always use current chart data and check for local updates.

Deviation Explained

Deviation is the compass error caused by magnetic fields within the aircraft, such as those from electrical equipment, metal structures, or even portable devices. Deviation varies with aircraft heading and can also change if new equipment is installed or the aircraft's magnetic environment is altered. To manage deviation, the compass is 'swung' and a deviation card is produced, showing corrections for various headings.

Applying Variation and Deviation

To determine the correct heading:

• Start with the true course from your chart.
• Apply variation to get the magnetic course.
• Apply deviation (from the compass card) to get the compass heading to steer.

Remember: "True, Variation, Magnetic, Deviation, Compass" (TVMDC) is the standard order for conversions.

Causes of Variation and Deviation

• Variation: Caused by Earth's magnetic field not aligning with the geographic poles; varies by location and time.
• Deviation: Caused by magnetic influences inside the aircraft; varies by heading and aircraft configuration.

Variation and Deviation on Charts

Charts display variation at specific locations, often with annual change rates. Always interpret charted headings and bearings in light of local variation.