Magnetic Declination Explained

Magnetic declination is the angle between magnetic north (where a compass needle points) and true north (the geographic North Pole). It varies by location — about −13° in New York and near 0° in London in 2026 — and changes as the north magnetic pole drifts ~55-60 km per year.

Magnetic declination is the correction that makes a compass-derived bearing into a true-north bearing. It is also one of the few quantities in navigation that changes continuously with both position and time, requiring a model rather than a fixed table to compute. For the historical compass-vs-GPS transition see /learn/celestial-navigation and /learn/how-gps-works; for the wider true-north reference see /learn/the-prime-meridian. This pillar runs the physics behind the variation, the World Magnetic Model that publishes it globally, the values for major cities, the drift of the magnetic poles, and the operational implications for hiking, aviation runway numbering, and survey work. Supports cover /learn/magnetic-vs-true-north, /learn/the-world-magnetic-model and /learn/why-magnetic-north-moves.

The geometry

A magnetic compass needle aligns with the local horizontal component of Earth's magnetic field. That direction is not the same as true north (toward the geographic North Pole) because:

The total angle between magnetic north and true north at a point is declination. Positive declination means the compass needle points east of true; negative means west. The convention varies between older British/USGS texts (east positive) and some maritime sources (west positive). The WMM and modern aviation use east-positive.

Declination across the continental US (2026)

The Continental US spans roughly 17° of declination from west to east, illustrating the typical mid-continent variation pattern.

The line where declination equals exactly 0 — the agonic line — runs roughly through the Great Plains in 2026 (Wisconsin → eastern Iowa → eastern Kansas → eastern Texas). West of it, declination is east-positive; east of it, west-negative. In 2000 the agonic line passed through Lake Michigan; the line has moved several hundred kilometres westward since.

Global highlights

Outside North America, declination varies even more widely.

The South Atlantic Anomaly — a region of weak magnetic field extending from southern Africa through South America — produces unusually large westward declinations there (often −25° or more). This is also where satellite electronics are most vulnerable to high-energy particles entering the field, because the field is weaker and the trapped-radiation belts are closer to Earth's surface.

The drift of the magnetic poles

Earth's magnetic field has been drifting and changing since records began — and the drift has accelerated dramatically in recent decades.

The recent acceleration is large enough that the WMM, which had been updated every 5 years, needed an out-of-cycle update in 2019 (the WMM 2019 release) because the 2015 model was no longer accurate enough for high-Arctic navigation. The next regular update is WMM 2025 (epoch 2025.0).

The south magnetic pole drifts much more slowly — about 10-15 km/year — and has been near 64° S, 138° E (off the coast of Antarctica) for decades.

Why declination matters operationally

Aviation has the most visible consequence: runway numbers are magnetic bearings, and when declination changes enough to shift the magnetic bearing across the rounding threshold (e.g., from 9.4° to 10.6° → runway changes from 09 to 11), the runway has to be physically renumbered. Major airports renumber once every 30-60 years on average.

The World Magnetic Model

The World Magnetic Model (WMM) is the canonical global declination model, published jointly by NOAA NCEI and the British Geological Survey.

The model is spherical-harmonic: 195 coefficients (Gauss coefficients) and their first time derivatives, computed against several years of satellite and ground-station data, predict declination/inclination/intensity at any point on Earth for any time in the 5-year window. The coefficient file is under 10 KB and runs in any language with basic math libraries.

The IGRF (International Geomagnetic Reference Field) is the scientific equivalent — updated every 5 years by an international working group (IAGA V-MOD) and used as the consensus reference for paleomagnetic, satellite-mission and academic work. WMM and IGRF agree to within a few arcminutes at any point.

Sign convention: east positive

There are two conventions in operational use; the modern aviation and WMM choice is east-positive.

The navigation mnemonic "TVMDC" (True → Variation → Magnetic → Deviation → Compass, "Add East" applied westbound) handles the operational conversion. For software, the cleanest rule: store declination signed with east-positive WMM convention; convert at the display layer.

Common misconceptions