Magnetic North vs True North (and Declination)

When you use a compass seriously, reading a map, plotting a route, you quickly meet three different “norths.” Understanding them prevents surprisingly large navigation errors.

The three norths

• True north (geographic north) is the direction of the geographic North Pole, where all the lines of longitude meet. Maps and GPS are based on true north.
• Magnetic north is the direction your compass needle actually points, toward Earth’s constantly shifting magnetic pole in the Arctic. It is not the same place as the geographic pole.
• Grid north is the direction of the vertical grid lines on a printed map projection. Its difference from true north is grid convergence and depends on the projection and location. Use the map’s reference information when plotting a route.

For most people the important distinction is between magnetic north (what the compass shows) and true north (what the map uses).

What is magnetic declination?

Magnetic declination (also called magnetic variation) is the angle between magnetic north and true north at your location. It is described as east or west:

• East (positive) declination means magnetic north lies to the east of true north.
• West (negative) declination means magnetic north lies to the west of true north.

Declination changes depending on where you are on Earth, it can be zero on some lines and exceed 20° in parts of North America, and it drifts slowly year to year as the magnetic pole moves.

How to convert between them

To turn a magnetic bearing into a true bearing:

True = Magnetic + East declination · True = Magnetic − West declination

For example, if your compass reads a magnetic bearing of 90° and your local declination is 10° east, the true bearing is 100°. If declination were 10° west, the true bearing would be 80°.

Online Compass does this for you: tap True north, and it calculates your local declination from the current World Magnetic Model, then shows a geographically corrected heading automatically.

Why iPhones and Androids can differ

Safari exposes a non-standard webkitCompassHeading. WebKit’s implementation supplies magnetic heading, not a promise of true north. Supported Android browsers can expose an absolute-orientation event, from which this tool derives a magnetic heading while the phone is held flat. Online Compass applies a location-based declination correction to both paths for true north. Browser support and sensor quality still vary; changing the reference cannot remove magnetic interference.

When it actually matters

Over short, casual distances a few degrees of declination is negligible. But over long distances it compounds: an error of just 5° over 10 kilometres puts you nearly a kilometre off course. For hiking, sailing, or any map-and-compass navigation, always check the declination for your area and convert your bearings, or let the compass do it for you.

Sources and implementation limits

• NOAA World Magnetic Model: the magnetic model used to estimate declination.
• WebKit’s iOS motion implementation: the browser obtains magneticHeading for its compass event.
• MDN DeviceOrientationEvent: sensor properties and browser compatibility.

The model describes Earth’s magnetic field, not the field around a nearby speaker, car or steel cupboard. Our accuracy and testing guide separates software checks from physical-device accuracy measurements.