Guides

How GPS works on a phone, why the numbers sometimes disagree, and what to do about it. Written by the people who make Vane, with sources linked.

Tools

• Online speedometerA free online GPS speedometer that runs in your phone's browser. Shows current speed, top speed, average and distance in km/h, mph or knots. No app, no sign-up.
• Online compassA free online compass that runs in your phone's browser. Shows your heading in degrees and points, from the phone's magnetometer. No app, no sign-up.

Guides

• Choosing a speedometer app for iPhoneWhat makes a good iPhone speedometer app, from 7,688 App Store reviews of speedometer, compass and altimeter apps: no ads over the reading, honest readings, works offline, no subscription traps, Apple Watch and a head-up display.
• GPS speed vs car speedometerWhy a car's speedometer usually reads a few percent higher than GPS: the UN R39 rule that forbids reading low, tyre wear and size, and when the GPS is the one that is wrong.
• GPS speed on a trainSpeedometer apps often show 0 or jump around on trains. Why: metal carriages, coated windows, tunnels and cuttings. How to get a steady reading: window seat, phone at the glass, and what to expect from the numbers.
• Plane speed and altitude on iPhoneYes, your iPhone's GPS can show your plane's ground speed and position in flight, in airplane mode, from a window seat. What to expect, why the speed can exceed 1,000 km/h, and why the altimeter shows the cabin, not the plane.
• Why your phone compass is wrongWhy a phone compass points the wrong way: magnets, MagSafe cases, car mounts, steel, calibration and true vs magnetic north. How to check it and fix it, step by step.
• Mileage log for taxes, 2026 ratesWhat a mileage log must contain for the IRS, HMRC, CRA and ATO, the 2026 business mileage rates (including the IRS mid-year change to 76 cents), and how to keep one with a phone.
• Open a GPX file on iPhoneWhat a GPX file is, what is inside it (waypoints, routes and tracks), and how to open, view and follow one on an iPhone, step by step.
• Setting a phone anchor alarmHow a GPS anchor alarm works, how much scope to let out, how to work out the swing radius, where to mark the anchor, and how to stop a phone alarm from failing in the night.

How it works

• How GPS measures speedA GPS receiver measures speed from the Doppler shift of satellite signals, not by timing how far you moved. Why that is more accurate, how accurate phone GPS speed really is, and when it goes wrong.
• How accurate is phone GPS?Phone GPS is typically accurate to about 5 m in open sky, worse near buildings and trees. What the accuracy circle means, what degrades it, which iPhones have dual-frequency GPS, and how accurate speed and altitude are.
• Does GPS work without internet?Yes: GPS is receive-only and works with no mobile data, no Wi-Fi and in airplane mode. What does need the internet is the map, and a faster first fix. How to use your phone's GPS offline.
• Why GPS stops in tunnelsWhy phone speedometers freeze, drop to zero or jump in tunnels, car parks and urban canyons, what car navigation systems use instead, and why a phone cannot simply carry on from its motion sensors.
• Barometric vs GPS altitudeHow a phone's barometric altimeter works, why it drifts with the weather, why GPS altitude is noisier, and how to calibrate for accurate elevation on a hike, a ski day or a flight.
• Coordinate formats and converterConvert GPS coordinates between decimal degrees, degrees-minutes, degrees-minutes-seconds, UTM and MGRS, and learn how each format works and how precise each digit is.
• Speed units and converterConvert knots, mph, km/h and m/s instantly, with the exact definitions, a quick-reference table, and why boats and planes use knots.