MinimalistHow your phone measures distance with GPS
Your phone measures distance by asking “where am I?” over and over, roughly every second, and adding up the straight-line gaps between each answer. GPS supplies the answers; the tracking app connects the dots and does the arithmetic.
That sounds simple, and at heart it is. But knowing what happens between the satellites and the number on your screen makes it much easier to trust your stats, and to understand the occasional odd result.
Where your location comes from
Step 1: The satellites send a time signal
GPS satellites orbit about 20,000 km above the Earth. Each one carries a very precise clock and continuously broadcasts a simple message: which satellite it is, where it is in its orbit, and exactly what time it sent the message.
Your phone doesn’t send anything back. It only listens. That’s an important detail: GPS itself is a one-way, receive-only system, and it works without a data connection or a phone signal.
Today, “GPS” is shorthand for several systems working together. The American GPS constellation is the best known, but modern iPhones can also listen to others, such as Europe’s Galileo and Russia’s GLONASS. More satellites in view usually means a steadier position.
Step 2: Timing becomes distance
Radio signals travel at the speed of light. When your phone receives a satellite’s message, it compares the time the message was sent with the time it arrived. The tiny delay, a fraction of a second, tells the phone how far away that satellite is.
One satellite tells you that you’re somewhere on a huge sphere around it. Two narrow it down to a circle. Three narrow it to a point or two. In practice the phone needs at least four satellites, because the fourth one lets it correct for the fact that its own clock is far less precise than the atomic clocks in orbit. This process is called trilateration.
The result is a location fix: a latitude, a longitude, usually an altitude, and an estimate of how accurate that fix is likely to be.
Step 3: The phone gets some help
A pure GPS receiver starting cold can take a while to find satellites. Phones speed this up with what’s often called assisted GPS. Using nearby Wi-Fi networks and cell towers, the phone gets a rough idea of where it is and which satellites should be overhead, so it can lock on faster.
The iPhone also blends in data from its motion sensors, like the accelerometer and gyroscope, to smooth out the track between fixes. You don’t see any of this. You just see a blue dot that settles quickly.
Under open sky, a good phone fix is typically accurate to within a few meters. Between tall buildings, under dense trees, or indoors, it gets worse, sometimes much worse. More on that in our guide to why your GPS distance can seem off.
How the app turns dots into distance
Here’s where distance comes from. While you walk or run, a tracking app records a stream of fixes, each with a time stamp. Picture a trail of breadcrumbs on a map.
To get distance, the app measures the gap between each pair of neighboring points and adds them all up. Because the Earth is round, it doesn’t use a flat ruler; it uses a formula that calculates distance across the curved surface between two coordinates. Over a few meters the difference is tiny, but it’s the correct way to do it.
A simplified example of a few seconds of walking:
| Time | Fix | Gap from previous | Running total |
|---|---|---|---|
| 0 s | Point A | — | 0.0 m |
| 1 s | Point B | 1.3 m | 1.3 m |
| 2 s | Point C | 1.4 m | 2.7 m |
| 3 s | Point D | 1.4 m | 4.1 m |
Do that a few thousand times and you have a 5 km walk.
Filtering out the noise
Raw GPS points wobble. Even standing perfectly still, your reported position can drift by a few meters as the satellite geometry shifts. If an app added every tiny wobble to your total, a coffee stop could add distance you never walked.
So trackers filter. Common approaches include ignoring fixes that report poor accuracy, ignoring movement that’s smaller than the expected error, and discarding points that would imply an impossible jump, like 50 meters in one second on a walk. Every app tunes this differently, which is one reason two apps on the same walk rarely agree to the meter.
There’s a trade-off here. Filter too little and you collect “phantom” distance from drift. Filter too much and you cut corners off a winding path, so your distance comes out a little short.
How pace, speed and moving time are worked out
Once an app has distance and time stamps, the rest is division:
- Speed = distance ÷ time, shown as km/h or mph.
- Pace = time ÷ distance, shown as minutes per kilometer or per mile. Walkers and runners often prefer pace because it maps directly to “how long will this take?” Our explainer on pace vs speed goes deeper.
- Moving time leaves out the moments you were paused, so a long chat at a crossing doesn’t drag down your average.
Minimalist, for example, records your route on a map along with distance, moving time, average pace and average speed, and you can pause and resume if you stop for a while. All of that is derived from the same trail of location fixes.
What GPS distance doesn’t count
A few quirks are worth knowing:
- Hills are mostly flattened. Most trackers measure horizontal distance across the map. A path up a steep slope is a bit longer than its map footprint, but on everyday routes the difference is small. On a 10% grade, it’s about half a percent.
- Treadmills and indoor tracks don’t work. If your position isn’t changing on the map, GPS sees no distance. For indoor walking, step counting is the better tool; see pedometer vs GPS.
- Time between fixes matters. If the signal drops for 30 seconds in an underpass, the app draws a straight line between the last good point and the next one. On a straight street that’s fine. On a bendy path it trims distance.
How accurate should you expect it to be?
For a typical outdoor walk with a reasonably clear sky, a phone-based GPS distance is usually close enough that the error doesn’t change anything meaningful: a few percent at most, and often less. The longer the route, the more the small point-to-point errors tend to even out.
If you want to check, walk a known distance. Four laps of the inside lane of a standard 400-meter running track is 1,600 meters, just short of a mile. Compare that to what your phone shows. If you’re within a few percent, your setup is working well.
FAQ
Does GPS distance tracking use mobile data?
The GPS signal itself doesn’t. Your phone may use Wi-Fi or cellular data to get a faster first fix, and map backgrounds may need data to load, but the position fixes that create your distance come from satellites.
Why does my distance keep growing when I’m standing still?
That’s GPS drift: your reported position wanders by a few meters. Good filtering hides most of it, but pausing your tracker during long stops is the simplest fix.
Is GPS more accurate for running than walking?
Not really. The positioning is the same. Faster movement makes the gaps between fixes larger relative to the wobble, so drift matters slightly less, but both walking and running distances are generally reliable outdoors.