Magnetic declination, explained

There are two norths in common use and they are not in the same place. Everything awkward about compass navigation comes from that one fact.

True north is the direction of the geographic north pole, the point the earth spins around. It is the north on every map, every site plan, every satellite azimuth and every property boundary. It does not move.

Magnetic north is the direction a magnetised needle settles into. It is governed by the earth’s magnetic field, which is generated by convection in the molten outer core, and it is not aligned with the spin axis. It also wanders: the north magnetic pole has moved hundreds of kilometres over the last century and has been accelerating.

The angle between the two, measured from where you happen to be standing, is magnetic declination, also called magnetic variation on charts. It is the correction that turns one into the other.

It depends on where you are

This is the part that catches people. Declination is not a global constant you can look up once. It is a function of position, because it is the angle between two directions that both depend on where you are standing.

Stand on a line running roughly through the middle of North America and it is near zero: the two norths line up from there. Move to the Pacific Northwest and it is fifteen degrees or more east. Move to the eastern seaboard and it is west instead. Parts of Alaska exceed twenty degrees.

Fifteen degrees does not sound like much. Walk a kilometre on a bearing that is fifteen degrees off and you arrive about 260 metres from where you intended. Walk five kilometres and it is more than a kilometre and a quarter.

Which one you want

The rule is simpler than the arithmetic suggests. Ask where the direction came from.

Use true north when the direction came from a map. Site plans, property boundaries, solar panel azimuths, satellite dish azimuths, the qibla, planning drawings, anything with coordinates. All of these are specified against the map, and the map means true.

Use magnetic north when the direction came from a compass. A bearing you took with a baseplate compass, or one printed as magnetic on a nautical chart. Stay in magnetic and do not correct something that was never in true to begin with.

The correction

Going from a true bearing to the magnetic bearing you will actually steer:

• East declination: subtract.
• West declination: add.

Going the other way, from a magnetic reading to a true bearing, reverse both. The old mnemonic is east is least, west is best, meaning east declination is subtracted and west is added when converting true to magnetic.

The failure here is not forgetting the correction. It is applying it backwards, which does not leave you with the original error. It doubles it. Someone in a region with twelve degrees of declination who corrects the wrong way is now twenty-four degrees out, worse off than if they had done nothing.

Which is a reasonable argument for letting software do it, provided the software tells you which direction it is reporting. A compass that shows a number without saying whether it is true or magnetic is not saving you from this problem, it is hiding it.

The third north

For completeness, because it appears the moment you put a compass on a gridded map: grid north is the direction of the northward grid lines on a projected map, and it differs from true north by an amount called grid convergence.

On a national grid it is usually under two degrees, and most walkers correctly ignore it. It matters at long range, near the edges of a grid zone, and in survey work. Mostly it is a footnote, but it explains why careful navigation texts talk about three norths rather than two.

It changes

Declination is not fixed in time either. The field drifts, and published values carry an epoch and an annual rate of change for that reason.

Practically, this means a declination figure printed on a map from the 1990s is out of date, sometimes by several degrees. Models like the World Magnetic Model are revised every five years, and the current one is the figure you want. If you are correcting a bearing that matters, get a current value for your position rather than one from the margin of an old sheet.