Getting an accurate heading from your phone
A phone compass reads the Earth's magnetic field with a magnetometer, and that field is weak. Anything magnetic nearby is louder than the planet, which is why a compass that was perfect outdoors can be forty degrees wrong at your desk.
Two different norths
Magnetic north is where the field points; true north is the axis the Earth turns on. The angle between them is the declination, and it depends entirely on where you stand — negligible in some places, well over ten degrees in others. Maps are drawn to true north, so if you are navigating from a map you want a compass that applies the declination for your location rather than the raw magnetic heading.
The figure-eight calibration
Magnetometers drift, and they are also fooled by any magnetism the phone itself has picked up. Waving the phone in a figure-eight pattern shows the sensor the field from many orientations at once, which lets it solve for and subtract its own offset. Do it whenever the reading looks implausible, and do it away from the desk.
What is pulling your heading
Magnetic case closures and card-holder cases are the worst offenders and they never leave, so the error follows you everywhere. Speakers, laptops, car dashboards, steel desks, reinforced concrete, radiators and power cables all bend the field too. Step a couple of metres away from metal, hold the phone flat and level, and give the reading a second to settle before trusting it.
Frequently asked questions
Do all phones have a compass?
No. The magnetometer is a separate sensor and cheaper models sometimes skip it. Without one, apps can only show a heading derived from movement, which needs you to be walking.
Why does the compass ask me to calibrate so often?
Usually a magnetic case or a nearby magnet keeps re-corrupting the offset. If the request keeps coming back, suspect the case before the phone.
Should I use magnetic or true north?
True north for map work, magnetic north when you are following a bearing given by another magnetic compass. The important thing is knowing which one your app is showing.
Sixteen tools, one app. TOOLBOX turns the sensors already inside your phone into a level, an inclinometer, a screen ruler, a caliper, a compass, a decibel meter and a strobe tachometer. It runs in the browser, with no account and nothing to install.
Open TOOLBOX