Device Monitor iPhone compass: true north vs magnetic north, and how to get an accurate heading
Device Monitor's Digital Compass shows Magnetic Heading and True Heading side by side, plus the Accuracy iOS puts on the reading. Use true north for maps and most outdoor use; magnetic north only when you're matching a paper compass or a chart given in magnetic bearings.
True north or magnetic north: which should you use?
True north points at the geographic North Pole, the north printed at the top of every map. Magnetic north is wherever a compass needle actually points, and it drifts away from true north by an amount called declination - how much depends on where you stand, and it can run to several degrees or more.
- Use true north with digital maps, when following directions from a map app, or when checking which way a window or balcony faces.
- Use magnetic north when you compare against a traditional needle compass or a bearing someone gave you from one.
How to read both headings in Device Monitor
- Open Diagnostics > Environment Sensors > Digital Compass.
- Allow location when it asks - true heading needs your position to work out local declination; without it only the magnetic heading is reliable.
- Hold the phone flat, screen up, top pointing wherever you're measuring.
- Read the big dial for the heading in whole degrees, plus the direction word underneath.
- Scroll to Heading Details for Magnetic Heading, True Heading, Accuracy and Direction.
A worked example
| Magnetic heading from the sensor | Dial | Magnetic Heading row | Direction |
|---|---|---|---|
| 200.4° | 200° | 200.4° | South |
| 47.6° | 48° | 47.6° | Northeast |
| 352.8° | 353° | 352.8° | North |
The direction word comes from eight 45° sectors centred on the compass points - North covers 337.5° to 22.5°, Northeast 22.5° to 67.5°, and so on round the dial. The dial itself rounds to whole degrees while Heading Details keeps one decimal.
What the accuracy figure means
Accuracy is iOS's own guess at how far off the heading might be, in plus-or-minus degrees. Smaller is better - a few degrees is fine for everyday use, and a big number is the app telling you to calibrate, or that something nearby is messing with the reading.
iPhone compass wrong? How to calibrate it
- Step away from metal and magnets - laptops, speakers, car dashboards, steel tables, magnetic phone cases and mounts all pull the reading off.
- Take off any case with a magnet or a metal plate.
- Move the phone in a slow figure-8 in the air, turning your wrist as you go, the way the app's own tips suggest.
- Watch Accuracy in Heading Details - once it drops, trust the heading more.
Questions and answers
Why does the true heading not appear?
Is the compass free?
Does the app send my location anywhere?
Why is my iPhone compass off by several degrees?
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