GPS Distance and Accuracy
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GPS gives good-enough distance and pace outdoors, but multipath, tree cover, and tunnels introduce error — and vertical (elevation) error is worse than horizontal.
Detail
GPS (more precisely GNSS — GPS, Galileo, GLONASS, BeiDou) fixes your position from satellite signals; a watch logs a track of points and sums the segments into distance, from which pace/speed is derived (see cardio-pace). Outdoors with a clear sky, horizontal accuracy is typically within a few meters and distance is reliable for training.
Where error creeps in:
- Multipath — signals bouncing off buildings ("urban canyon"), cliffs, or water inflate or scatter the track, overstating distance.
- Tree cover, tunnels, indoors — weak/blocked signal causes drift or dropout.
- Sampling and smoothing — low logging rates cut corners on twisty routes (understating distance); aggressive smoothing can do the opposite.
- Vertical error is larger than horizontal, so GPS elevation is the least reliable channel — a barometric altimeter is preferred for elevation-gain.
Consequences for tracking:
- Treat outdoor distance/pace as good but not exact; don't compare GPS pace to treadmill or measured-track pace as if identical.
- Indoors or with poor signal, prefer a footpod, wheel/odometer, or machine distance, or intensity metrics that don't need GPS — heart rate or power.
- A "GPS pace spike" through a tunnel or downtown is usually signal error, not a real surge.
This sits alongside the general caution that all device-derived metrics carry error (see wearable-accuracy-limits).
Key facts
- GNSS distance/pace is reliable outdoors with clear sky (within a few meters horizontally).
- Error sources: urban multipath, tree cover, tunnels, indoors, low sampling, smoothing.
- Vertical (elevation) error exceeds horizontal — use a barometric altimeter for climb.
- Don't equate GPS pace with treadmill/track pace.
- Poor signal → use footpod/wheel/machine distance or HR/power instead.
Connections
- cardio-pace — distance/pace are GPS-derived and inherit its error.
- elevation-gain — why barometric beats GPS for vertical.
- power-ftp — the GPS-independent intensity alternative.
- wearable-accuracy-limits — broader accuracy context.
- what-to-log-cardio — GPS underlies several logged fields.
Educational content only — not medical advice. Always consult a qualified professional for individualized guidance, especially around injury, pregnancy, or medical conditions.