Why your GPS watch distance is wrong

You finish a run with a friend. You started and stopped together, ran the same pavements, and yet your watches disagree by a few hundred metres over a long loop. Or you race a certified 10K and your watch insists you covered 10.15km. It is one of the most common questions runners ask, and the reassuring answer is that a small gap is almost always normal. Here is what is really going on, and the simple habits that pull your numbers closer to reality.

Why is GPS distance never exact?

Your watch works out where you are by timing faint signals from satellites orbiting thousands of kilometres away. Every fix it takes carries a margin of error, often a few metres in the open and a good deal more in a tricky spot. On top of that, your watch does not trace a smooth curve. It records your position at intervals, usually about once a second, then joins those dots with straight segments. Add up thousands of tiny position errors and slightly clipped corners over an hour of running, and a difference of one or two percent is completely ordinary.

What scrambles the signal on a run?

Anything between your wrist and a clear view of the sky makes the watch work harder. The usual culprits:

• Tall buildings. In a built-up street, sometimes called an urban canyon, the signal bounces off glass and concrete before it reaches you. The watch can mistake a reflected signal for a direct one, a problem known as multipath, and your track drifts across the road or straight through a building.
• Tree cover. A dense canopy weakens the signal, so the watch has fewer satellites to work with and the position wanders. This is why a wooded loop can read short one week and long the next.
• Tunnels, underpasses and bridges. Solid structures block the sky entirely, so the watch effectively guesses across the gap until it locks on again.
• Your arm and wrist. Your own body blocks part of the sky. A watch buried under a jacket sleeve, or an arm held tight to your side, gets a weaker signal than one worn on top of a sleeve.
• Starting too soon. If you press go before the watch has a firm lock, those first few hundred metres are its worst guess of the whole run.

Why do you and a friend get different distances on the same run?

You ran shoulder to shoulder, so how can the totals differ? Because each watch is making its own independent estimate. Different models use different chipsets, sample at different rates and run different smoothing algorithms, so they round corners and cope with a weak signal in their own way. One watch might sit on the arm nearest the buildings while the other faces open sky. None of that means one is faulty. They are simply two estimates of the same route, and estimates vary. It is the same reason two watches can log slightly different climbs on an identical hill. The gap tends to grow on longer runs, because a steady one or two percent error stacks up over more kilometres, and it grows again in town, where reflected signals push each watch off in its own direction.

Why does a race measure longer than the official distance?

This one catches almost everyone. Certified courses are measured along the shortest route you are allowed to run, hugging the inside of every bend right by the kerb, and measurers even add a tiny safety margin so the course can never come up short. In real life you drift wide on corners, weave around other runners, cross to a water station and back, and swing out to overtake. Every one of those metres is extra distance your watch faithfully records. Combine that with normal GPS drift on a crowded, often built-up route, and reading a little long is exactly what you should expect. It does not mean the course was mismeasured. If anything, a watch reading a fraction over the advertised distance is a sign the measurement did its job, since the certified line is the tightest one physically possible.

Common causes, and what helps

How can I get more accurate distance data?

You cannot beat physics, but a few habits make a real difference:

• Wait for a good signal lock. Give the watch a minute outside with a clear view of the sky before you start. Standing still until it confirms a strong fix beats setting off early and letting it catch up.
• Use multi-band or dual-frequency GNSS if your watch offers it. Newer chipsets track satellites on two frequencies at once, which cuts the multipath errors that plague built-up areas. It can use more battery, so many members keep it for races and key sessions.
• Keep firmware and satellite data updated. Watches download a short forecast of where the satellites will be, sometimes labelled EPO, so they lock on faster. Syncing regularly keeps that data fresh.
• Mind your arm position. Wear the watch over a sleeve rather than under it, and it will hear the sky more clearly.
• Avoid starting under cover. A start beneath trees, a bridge or a tall building hands the watch its hardest job at the moment it matters most. Step into the open first if you can.
• Use a foot pod or treadmill calibration indoors. When there is no signal at all, a foot pod works out distance from your stride instead. Calibrating it, or the treadmill, over a known distance keeps indoor runs honest.

So is my watch actually broken?

Almost certainly not. A one or two percent difference between two watches, or between your watch and a race distance, is normal GPS error rather than a fault. A watch only really deserves a closer look if the distance is wildly off every single time, the track jumps far from roads you know you ran, or it never seems to get a lock at all. The rest of the time, the wrong number is just GPS being GPS. What matters for training is consistency: if you always measure your favourite loop the same way, you can trust the trend even when the exact figure drifts by a few metres.

That is really the point. One slightly long parkrun or one odd reading in the woods tells you very little on its own. Weeks of runs measured the same way tell you plenty. Edge works alongside the watch you already own and turns your runs, strength and HIIT into one connected plan, so a single quirky data point never throws off the bigger picture. It is how 18,000+ members train across running, strength, HIIT and mobility without obsessing over every decimal place.

This article is general guidance, not medical advice. If you are new to exercise, returning from injury, pregnant, or managing a health condition, speak to your GP before starting, and see a physio for any niggle that does not settle.