You open a navigation app, follow the blue arrow, and end up walking the wrong direction for two blocks before the map catches up. Sound familiar? The culprit is often not bad GPS — it's a distorted magnetic field throwing off your phone's magnetometer, the sensor that powers every compass, map heading, and augmented-reality overlay on your device.

Modern life is packed with magnetic interference. The good news: once you know what to look for, it's easy to diagnose and fix.

Apple's MagSafe system uses a ring of magnets embedded in the back of iPhone 12 and later models, and in compatible cases and accessories. Near a MagSafe charger, the magnetic field measured at the phone's sensor can spike to 200–500+ microtesla (µT). The Earth's natural magnetic field is only 25–65 µT depending on where you live — so a charging pad can flood your sensor with a field that is five to ten times stronger than the planet itself.

MagSafe cases alone add a moderate amount of static magnetism. It's enough to shift compass headings by 10–20 degrees if the phone hasn't been recalibrated since you snapped the case on.

The inexpensive disc-and-ring car mounts — where you stick a metal plate to your phone and attach it to a magnetic ball-socket — place a strong ring magnet directly against the back of your device. Field readings inside a car with one of these mounts can exceed 300–600 µT, making the compass essentially useless. Navigation apps will spin, AR overlays will point at ceilings, and heading-lock features will drift continuously.

The fix is straightforward: switch to a non-magnetic mount that grips the phone mechanically, such as a clamp-style vent or dashboard mount. If you use MagSafe-compatible accessories in the car, the magnet ring is weaker and farther from the sensor than the disc mounts, but you should still recalibrate after attaching the phone.

Ferromagnetic metals — steel, iron, some alloys — distort the local field even without being magnetized themselves. This is called soft iron interference: the material reshapes the field lines around it, bending the direction your magnetometer reads.

A steel desk, a laptop chassis, large speakers, or even a refrigerator can shift readings by 5–30 degrees when your phone is lying flat on or near them. The phone doesn't have to be on top of the object; proximity of even a few centimetres is enough for thicker metal surfaces.

Hard iron interference is different — it comes from permanently magnetized objects like loudspeaker magnets, magnetic knife strips, or magnetized phone accessories. Hard iron errors are persistent: they shift the compass by a fixed amount in every direction and don't go away by moving the phone around the object.

The clearest sign is a compass heading that drifts when you hold your phone still, or a heading that snaps to a different value as soon as you lift the phone off a surface. A more precise way to check is to look at the raw magnetic field strength in microtesla. Values outside the 25–65 µT window are a strong signal that something is distorting the field.

NorthPin True North Compass (also on Android) shows your live µT reading on the main screen and raises a visual warning when the field strength falls outside the normal range. It's free and has no ads. If NorthPin shows 180 µT while your phone is sitting on your desk, you know exactly why your navigation has been acting strange.

Next time your map heading seems off, open NorthPin and glance at the µT number before blaming your GPS. Chances are something magnetic is sitting a lot closer than you think.