Your watch works out your pace by listening to satellites. Most of the time this works well. But on race day you often run where the signal gets confused: between tall buildings, under thick trees, through tunnels and under bridges. There the watch can suddenly show a pace that is far too fast or too slow, even though your speed has not changed. A good pacer knows when to trust the watch and when to ignore it.
What goes wrong, and where#
GPS (Global Positioning System) needs a clear view of the sky. Anything that blocks or bounces the signal makes your position less accurate, and because pace is calculated from changes in position, a small position error quickly becomes a big pace error.
| Place | What happens to the signal | What you may see on the watch |
|---|---|---|
| Between high-rises (an "urban canyon") | Signals bounce off glass and concrete before reaching your watch. | Pace jumps around; the track zig-zags; distance may grow too fast. |
| Under dense tree canopies | Leaves and branches weaken the signal, especially when wet. | Pace drifts slowly; distance may come up short. |
| Tunnels and underpasses | The signal is lost completely for a while. | Pace freezes or drops, then "catches up" with a jump when you come out. |
| Under and on bridges | The deck above or below blocks part of the sky; height changes are not counted. | Short pace spikes; distance slightly off on long climbs. |
Picture a race through central Ho Chi Minh City. Between tall office towers, your watch suddenly says you are 20 s/km too slow. If you speed up to "fix" it, your whole group surges for nothing, and your runners pay for it later.
Which numbers to trust#
The Pacer Support Guide explains the recommended data screen in Pacing with a watch. In short:
- Instant pace (your pace over the last few seconds) is the noisiest number. It can jump by 10–20 s/km even in good conditions. Keep it off your main screen.
- Lap pace (your average pace in the current kilometre) is your steering wheel. A short GPS glitch has less effect on it.
- Average pace (your overall pace since the start) is your compass. It changes slowly, so one bad stretch hardly moves it.
- Elapsed time at official km markers is the most reliable check of all, because the markers do not depend on satellites.
When the signal is poor, rely on your body: if your effort, breathing and cadence feel the same as a kilometre ago, your speed is probably the same too.
Set up your watch for the best signal#
You cannot remove buildings, but you can help your watch:
- Use the most accurate recording mode. On Garmin, switch from Smart Recording to Every Second recording (see Garmin settings).
- Use multi-band (dual-frequency) GPS if your watch has it. It usually copes better with tall buildings but uses more battery, so test it on a long run first.
- Switch on 3D Speed and 3D Distance on courses with bridges, if your watch offers it (see 3D Speed & Distance).
- Get a GPS lock early. Start the run activity at least 5 minutes before the gun, standing in an open area if possible.
- Learn the course. Know where the tall buildings, tunnels and bridges are.
If two pacers' watches disagree in one of these zones, follow the routine in When watches display different results: the leader's watch is the reference, and the next official marker settles the question.
Key takeaways#
- Tall buildings, tree cover, tunnels and bridges all distort GPS pace.
- Instant pace is too noisy for pacing; use lap pace and average pace instead.
- Official km markers and elapsed time are your most reliable check.
- Use accurate recording, multi-band GPS (if available) and an early GPS lock.
- In a bad GPS zone, hold your effort steady and wait for the next marker before correcting.
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