True north vs magnetic north: what the difference means.

There are two norths, and they are not in the same place. True north is the top of the globe, the point every line of longitude runs to. Magnetic north is wherever a compass needle actually pulls, and that spot is in the Arctic, well off the pole, and it moves a little every year.

For walking to the store, the gap does not matter. For calling in a direction, reading a bearing off a map, or telling someone which way a person ran, it can be off by enough to point at the wrong block. This is the difference, why it exists, and how to make sure the heading you read is the one you mean.

The short version

True north, also called geographic north, is a fixed point: the North Pole, the top of the axis the earth spins on. It never moves, and it is the north that maps are drawn to.

Magnetic north is where the planet's magnetic field pulls a compass needle. That point is not the North Pole. It sits in the Canadian Arctic, hundreds of miles south of the geographic pole, and it drifts over time as the molten core shifts. So a plain compass points at magnetic north, not at the top of your map, and the two only line up in a few places on earth.

The angle between the two, measured from where you are standing, is called magnetic declination. Knowing your local declination is what lets you turn a compass reading into a true bearing that matches a map.

Why the two norths differ

The earth behaves like a giant, slightly crooked bar magnet. Its magnetic field comes from the churning iron in the outer core, and the axis of that field does not line up with the axis the planet spins on. So the magnetic pole, the place the field points straight down, ends up offset from the geographic pole by a wide margin.

That magnetic pole is also restless. It has wandered for as long as we have tracked it, and in recent decades it has picked up speed, moving from the Canadian Arctic toward Siberia. This is why a paper map prints a declination value along with the year it was measured: the number slowly goes stale as the pole keeps moving. True north, by contrast, is geometry. It does not drift, because it is just the top of the spin axis.

Declination: the angle between them

Magnetic declination, sometimes called magnetic variation, is the heart of the whole thing: the angle, at your exact location, between true north and the direction your compass needle points. It is given as so many degrees east or west.

• East declination means magnetic north sits to the right of true north from where you stand, so your compass reads low and you add the value to get a true bearing.
• West declination means it sits to the left, so your compass reads high and you subtract.
• Near zero, along a line called the agonic line, the two norths line up and a compass points true with no correction.

How big is it? It depends entirely on where you are. In parts of the continental United States declination runs past 15 or even 20 degrees, and it climbs much higher near the poles. Twenty degrees is not a rounding error. On a long bearing it can put you a full street, or a full ridge, away from where you meant to be. That is the whole reason the distinction is worth understanding.

A third north: grid north

There is actually one more north worth naming. Grid north is the direction the vertical lines run on a projected map grid, like the UTM grid that MGRS is built on. Because a flat grid cannot perfectly wrap a round earth, those grid lines tilt very slightly away from true north, an angle called grid convergence. It is usually small, often a degree or less, but it is why precise land-navigation work distinguishes all three: true, grid, and magnetic. For most everyday use you only need to keep the first two straight, true and magnetic.

Which one your phone shows

Your iPhone has both a magnetometer, which senses the magnetic field, and your GPS location, which it can use to look up the declination and convert to true. So it can show you either north, and there is a setting for it.

In the built-in Compass app, go to Settings, then Compass, and you will find a "Use True North" toggle. Turn it on and the app reports headings relative to true north. Leave it off and it reports magnetic. The catch is that this only governs the stock Compass app, and a lot of people never know the toggle is there, so they read magnetic headings without realizing it.

A magnetometer also has to be treated with a little care. Metal, speakers, magnetic phone mounts, and car bodies all bend the field nearby, which is why your phone occasionally asks you to wave it in a figure eight to recalibrate. Step away from large metal, recalibrate when prompted, and the reading settles down.

When the difference matters

Most of the time you can ignore all of this. The difference starts to count the moment a direction has to mean the same thing to two people.

• Calling in a direction of travel. "Headed northbound" should match the map dispatch is looking at, and that map is true north. A magnetic reading off by 15 degrees can turn north into north-northwest.
• Working off a paper or topo map. Map bearings are true (or grid). To walk one with a plain compass you have to apply declination, or the line you follow drifts.
• Marine and aviation. Charts and headings have long, formal rules for handling variation, because at speed a small angle becomes a large miss.
• Search, rescue, and patrol. When you are guiding another unit by direction and distance, everyone has to be reading the same north for the numbers to line up.

The simplest fix is to take the math out of it. A device that knows your location can apply the declination for you and just show true. Then the heading you read out is already the one that matches the map, with nothing to add or subtract in your head. Knowing your direction is one piece of the picture; reading your full position, address and coordinates, is the rest of it, and getting another phone to your exact spot is the last step.

Common questions

What is the difference between true north and magnetic north?

True north is the geographic North Pole, the fixed top of the earth's spin axis, and it is the north maps are drawn to. Magnetic north is where a compass needle points, toward the magnetic pole in the Arctic, which sits hundreds of miles away and slowly drifts. The angle between them at your location is called declination.

What is magnetic declination?

Magnetic declination, also called variation, is the angle at your exact location between true north and the direction a compass points, given as degrees east or west. East means the compass reads low, so you add the value; west means it reads high, so you subtract. Along the agonic line it is near zero and a compass points true.

How far apart are true north and magnetic north?

It depends entirely on where you stand. In parts of the continental United States declination runs past 15 or even 20 degrees, and it climbs much higher near the poles. Twenty degrees is enough to put a long bearing a full street or ridge off, which is why the difference is worth correcting.

Should I use true north or magnetic north?

Use whichever matches the map or the person you are talking to. Paper maps, GPS, and dispatch all work in true north, so for calling in a direction or following a map bearing, true is usually what you want. Plain magnetic compass work needs declination applied to match a map.

Does my iPhone show true or magnetic north?

Either, depending on a setting. In the built-in Compass app, open Settings, then Compass, and turn on "Use True North" to report true headings; leave it off for magnetic. The toggle only governs the stock Compass app, so many people read magnetic headings without realizing it.

Why does my compass keep asking to be calibrated?

The magnetometer that reads the field is sensitive to nearby metal, speakers, magnetic mounts, and car bodies, which bend the field and throw off the heading. Stepping away from large metal and waving the phone in a figure eight when prompted recalibrates it, and the reading settles down.