What Is Magnetic Declination and Why It Matters

Magnetic declination is the angle between true north and magnetic north at a given place and time. It is the correction that turns a compass reading into a direction you can plot, and ignoring it is the most common reason a carefully followed bearing does not arrive where it should.

It is called variation in marine and aviation contexts. Same quantity, same correction, different word.

How much error are we talking about?

Enough that it dominates every other source of navigation error on foot.

1° of error

17 m off per kilometre travelled

5° of error

87 m off per kilometre

10° of error

175 m off per kilometre

20° of error

347 m off per kilometre

Declination in the continental United States currently runs from roughly 15° east in the Pacific Northwest to roughly 15° west in Maine. In Alaska it exceeds 20° east. Move to high latitudes and it becomes extreme: within a few hundred kilometres of the magnetic pole, declination can be anything at all.

Compare that against a well-executed pace count, which is good to a few percent, or a GPS fix good to 4 m. Declination error is an order of magnitude bigger than either, and unlike them it is entirely avoidable.

Why it varies by place

The Earth's magnetic field is generated by convection in the liquid iron outer core, and it is nothing like a neat bar magnet. It has a rough dipole shape with substantial irregularities, plus local distortions from magnetised rock in the crust.

The result is that declination changes continuously as you move. Lines of equal declination, called isogonic lines, are drawn on magnetic charts and they wander considerably. The line of zero declination, where true and magnetic north coincide, is called the agonic line, and it has been migrating steadily westward across North America for decades.

Local geology adds a further wrinkle. Iron-bearing rock, ore deposits and volcanic terrain can pull a needle several degrees off the regional value, which is why some map sheets carry a note warning of local magnetic disturbance. No model predicts these; only local observation catches them.

Why it varies by year

The field drifts. The magnetic north pole has moved hundreds of kilometres since it was first located, at times exceeding 50 km per year, and its speed has itself changed markedly over the last few decades.

For you this means one thing: a declination value has an expiry date.

The reference model is the World Magnetic Model (WMM), produced jointly by NOAA's National Centers for Environmental Information and the British Geological Survey. It is reissued every five years, with the current edition covering 2025 to 2030. Out-of-cycle updates have been issued before when the field moved faster than the model predicted.

Getting a current value

Three sources, in descending order of reliability:

1. A WMM-based calculator or app, given your position and today's date. This is what a phone compass app is doing internally when it displays true north. It is accurate to well under a degree for regional purposes.
2. The declination diagram on the map sheet, updated using the stated annual rate of change. Adequate for most work.
3. Local observation, taking a bearing on a known landmark whose true bearing you can compute from the map. Slow, but it is the only method that catches local magnetic anomalies.

Declination is not something to memorise for a region and reuse. It changes enough over a few hundred kilometres of travel that a value taken at the start of a long movement can be meaningfully wrong by the end.

Applying it

The conversion between true and magnetic azimuth is a single addition or subtraction:

Traditionally phrased as "east is least, west is best" for the true-to-magnetic direction.

Note that for military map work the number you usually want is not declination at all but the G-M angle, the angle between grid north and magnetic north, because you are plotting against grid lines rather than meridians. That includes the grid convergence as well as the declination. Both are given in the map's declination diagram, and the distinction is covered in grid north, magnetic north and true north.

Things that ruin a compass reading regardless

Declination is a correction for the Earth's field. It cannot correct for the field around the compass itself, and these are much larger effects at close range:

• Vehicles. Stand well clear, tens of metres, not a few paces.
• Weapons and steel. A rifle held across the body will deflect a needle.
• Electronics. Radios, phones and headphones contain magnets and current- carrying wire.
• Power lines and fences. Overhead lines and wire fencing distort locally.
• Belt buckles, watches, glasses frames. Small but at the distance a compass is held, not negligible.

A phone magnetometer is more susceptible than a needle compass, not less. It sits centimetres from speakers, a battery and a camera magnet, which is why compass apps ask you to wave the phone in a figure of eight: that is the calibration routine estimating and subtracting the phone's own magnetic signature.

Summary

Declination is the angle between true north and the compass needle. It varies by hundreds of kilometres of geography and by year, it is worth 17 m of lateral error per kilometre per degree, and it is the correction most likely to be skipped. Get a current value for where you actually are, apply it in one direction consistently, and keep steel away from the compass.