Why does a GPS track show an impossible top speed?

The short answer

A GPS track has no built-in speedometer. Every speed figure is worked out afterwards from two things every recorded point already has — a position and a timestamp — by taking the distance between two consecutive points and dividing it by the time between them. Total distance and moving time are built by adding that arithmetic up across an entire route, so one bad segment is a small fraction of a very long sum. Maximum speed is built the opposite way: it is whichever single division, out of thousands, came out largest. One point that lands even a little off, paired with a short time gap to its neighbour, is enough to produce a division that answers "impossible" instead of "normal," and nothing else in the recording has to be wrong for that to happen.

Why one division has nowhere to hide

Picture two consecutive points recorded a second apart. If the second point is genuinely twenty metres from the first, that is a plausible walking or running speed once converted to an hourly figure. If a brief loss of clean signal — the kind that produces an ordinary position spike on the map — puts that second point forty metres away instead of twenty, the distance figure only doubled, but it is still being divided by the same single second. A small, unremarkable error in position turns into a large, implausible number in speed, because dividing by a very short time window magnifies whatever sits on top of it.

Total distance never faces that same magnification. Summing thousands of point-to-point gaps, most of them accurate, means one inflated gap is diluted by everyone around it rather than reported on its own. Maximum speed skips the dilution entirely by design — it exists to report the single worst, or best, instant in the whole file, which is exactly the instant a stray point is most likely to produce.

What actually produces the one bad point

The causes are the same short-lived conditions behind any other GPS oddity, just landing at a moment where the math happens to be least forgiving of them:

• A reflected signal off a nearby surface — a parked car, a wet rock face, a metal roof — arriving a fraction of a second after the direct one and shifting a single fix further than it should have moved.
• A satellite briefly dropping behind a ridge, a building, or your own body as you turn, narrowing the spread of satellites the receiver is using for just that one fix.
• A position recalculating right after a short loss of the sky altogether — a stand of dense trees, an underpass, a narrow gap between buildings — and briefly overshooting before settling back to normal.

Any of these can nudge a single point a few extra metres off. Spread across the rest of the track that error would be unremarkable. Divided by the one or two seconds between two consecutive points, it is enough to produce a speed that no one on foot or a bicycle actually reached.

Where it tends to show up

A handful of moments account for most impossible top speed readings, because they are exactly where a receiver is most likely to be working with a weaker or briefly interrupted view of the sky: the first minute or so of a recording, before the receiver has had time to settle onto a stable fix; a stretch under heavy tree cover or between tall buildings, where multipath reflection is common; and the moment right after resuming from a pause, if the phone spent the pause somewhere its view of the sky was worse than while it was actually moving.

What it does and does not tell you about the rest of the recording

An impossible top speed is a statement about one instant, not a verdict on the whole walk or ride. Distance, moving time and pace are all totals built across the entire track, so the same stray point that wrecks a maximum speed reading typically shifts those figures by a negligible amount, if it shifts them at all. Treat maximum speed as the single most exposed number a GPS track can produce, and the rest of the recording as considerably sturdier than that one figure makes it look.

Where TrailCam fits in

TrailCam reports distance, moving time, pace and elevation gain for every recorded route — the same four totals, built across the whole track rather than picked out from a single instant, for a hike, a walk or a ride. It does not report a maximum speed figure. A route exported as GPX 1.1 or CSV, which is part of the optional TrailCam Pro upgrade, carries the same points a top-speed calculation elsewhere would use, so the effect described above is what you would be looking at if another service calculated one from the file. TrailCam is free to download.