Magnetic variation is the angular difference between true north (the Geographic North Pole) and magnetic north (where your compass actually points). In aviation, this difference matters because some navigation references use true north while others use magnetic north, and confusing the two can send you miles off course. The Geographic North Pole sits about 1,200 miles from the Magnetic North Pole, and that gap creates a measurable offset that changes depending on where you are on the planet.
Why True North and Magnetic North Differ
Earth’s magnetic field is generated by molten iron moving deep in the planet’s core. The magnetic poles produced by this process don’t line up with the geographic poles that define Earth’s axis of rotation. A compass needle aligns itself with the magnetic field, so it points toward the Magnetic North Pole rather than the Geographic North Pole shown on maps. The angle between those two directions, measured from your specific location, is magnetic variation (also called magnetic declination).
This angle isn’t fixed. It changes based on where you’re standing on Earth’s surface, and it shifts gradually over time as the magnetic pole drifts. In some parts of the world, your compass might point several degrees east of true north. In others, it points west. Along a narrow band called the agonic line, the two norths happen to align and variation is zero.
How Variation Appears on Charts
Aviation charts display magnetic variation using isogonic lines, which connect points on the map where the variation value is the same. If you’re flying along a 10°W isogonic line, every point beneath you has a magnetic variation of 10 degrees west, meaning your compass points 10 degrees west of true north. The agonic line, where variation is zero, runs roughly through the central United States, though its exact position shifts over time.
These lines are printed on VFR sectional charts and include the variation value and the year it was measured. Because the magnetic field changes slowly, the values printed on a chart can drift out of date between publications. The World Magnetic Model, maintained by NOAA’s National Centers for Environmental Information, is updated every five years to keep pace with these shifts. The current model expires on December 31, 2029.
What Uses Magnetic North and What Uses True North
One of the trickiest parts of magnetic variation is that not everything in aviation uses the same directional reference. Runway headings, airport wind reports (AWOS, ATIS, and tower winds), VOR stations, approach procedures, and airway tracks are all referenced to magnetic north. But VFR charts, terminal aerodrome forecasts (TAFs), METARs, and enroute winds aloft are referenced to true north.
This split means pilots need to convert between the two systems regularly during flight planning and in the air. If you plot a course on a VFR chart, you’re working in true north. Before you can fly that heading using your magnetic compass, you need to apply the local variation to get a magnetic heading.
Converting Between True and Magnetic
The conversion itself is straightforward. If the magnetic variation at your location is west, you add the variation value to your true course to get a magnetic course. If the variation is east, you subtract it. The classic mnemonic is “East is least, west is best,” meaning east variation is subtracted (least) and west variation is added (best).
For example, if your true course is 270° and the local variation is 12°W, your magnetic course is 282°. If the variation were 8°E instead, your magnetic course would be 262°. The math is simple, but getting the direction of the correction wrong flips the error to double its size, which is why the mnemonic exists.
It’s worth noting that magnetic variation is different from magnetic deviation. Variation is caused by Earth’s magnetic field and depends on your geographic location. Deviation is caused by magnetic interference inside your specific aircraft, from electrical systems, metal structures, or avionics. Deviation varies by heading and is corrected separately using a deviation card posted near the compass.
How Variation Changes Over Time
The Magnetic North Pole moves. Over the past century, it has drifted from northern Canada toward Siberia, and the rate of movement has accelerated in recent decades. This drift gradually changes the variation value at every point on Earth, which has real consequences for aviation infrastructure.
Runway numbers are based on magnetic heading, rounded to the nearest 10 degrees. When the magnetic pole shifts enough, a runway’s actual magnetic bearing no longer matches its painted number, and airports have to rename it. Fairbanks International Airport in Alaska renamed runway 1L-19R to 2L-20R in 2009 after the local magnetic shift crossed the rounding threshold. This kind of renaming happens at airports worldwide as the field continues to evolve.
For navigation charts and models, this drift is why the World Magnetic Model gets a fresh update every five years. Between updates, the model includes predictive coefficients to estimate ongoing changes, but the unpredictable nature of Earth’s core means these predictions lose accuracy over time.
Variation at Extreme Latitudes
Magnetic variation becomes increasingly problematic as you fly closer to the magnetic poles. Near the poles, variation changes rapidly over short distances, the values are poorly defined, and the horizontal component of Earth’s magnetic field (the part that actually makes a compass needle swing left or right) becomes very weak. A magnetic compass in these regions is sluggish, unreliable, and essentially useless for precise navigation.
The standard cutoff is around 75° latitude. Below that, pilots navigate using conventional true or magnetic headings. Above 75°, most long-range operations switch to a system called grid navigation. Instead of referencing true or magnetic north, grid navigation uses a fixed set of parallel lines drawn on a polar chart projection, aligned with the Greenwich meridian. All directions are measured clockwise from this grid reference, labeled in “grid degrees.” This system eliminates the problem of converging meridians and wildly shifting variation values near the pole. U.S. Air Force weather reconnaissance flights to the North Pole from Fairbanks have used polar grid navigation as a routine practice for decades.
Variation vs. Deviation in Practice
When you plan a flight, you start with a true course drawn on your chart. You apply magnetic variation to convert that to a magnetic course. Then you apply your aircraft’s compass deviation (from the deviation card) to get the compass heading you actually steer. The full sequence, often taught as “true, variation, magnetic, deviation, compass,” builds from the chart to the cockpit in logical steps.
Variation typically changes only when you fly far enough to cross into a different isogonic zone. Deviation stays with your aircraft and changes with heading. On a short cross-country flight, you might use a single variation value for the entire route. On a long flight crossing multiple isogonic lines, you’d update the variation at each waypoint or leg change to keep your magnetic course accurate.