GPS watch accuracy on track vs trails

If the GPS is getting confused on the track, for whatever reason, pair it to a Garmin footpod for track sessions, You can manually calibrate the footpod based on a known distance (ie 400m) and then switch off the GPS when running on the track.

Why would you need the pod on the track?

Just use the manual lap button on the watch. (You can set the watch to flash the lap time to you everytime you do this for a few second; might already be the default).

It is a good skill to practice even for running road race. City races are notoriously bad due to buildings or trees in parks. I've blown up for going too fast due to GPS under-reporting the current pace. It turned out to be GPS blind spot (the same one in the same race the year after). To be fair, I've also PR'ed in a 10k race where the GPS under-estimate the distance. The stopwatch time is still the only most reliable info on the watch. If the race is well-marked with km or mile signs, I just hit the button right when passing them and use the lap time.

uplate123 wrote:

I have a feeling it has to do with signal, . . .

This is the primary issue. The GPS signal received by the watch is more or less corrupted by reflections from nearby objects (think metal fences, bleacher, and such) and/or shadowed (think buildings, hills, and such). On a track whatever biases are induced tend to repeat lap after lap which biases the measured distance, that is, the accumulated errors are not zero mean. One can see this by plotting the GPS track and observing 1) that it usually doesn't follow the precise path that was run and 2) that it more or less follows the same path on each lap.

On the road where one is not covering the same ground over and over, the induced biases are more likely to be uncorrelated and zero mean. That said, GPS doesn't work particularly well in "urban canyons" and other difficult environments and people do see significant errors using GPS on the roads.

If you use the pod on the track, you can calibrate it over a known 400m distance, so that the pod reads exactly 400m. You generally only have to calibrate the pod once, unless you drastically change the location of the pod relative to the watch. Once calibrated, you turn off the GPS on the watch and let the watch pick up distance from the pod instead. Works a charm and you never have to think about it again.

Once these chips, or similar ones, are out, things will change.

https://spectrum.ieee.org/tech-talk/semiconductors/design/superaccurate-gps-chips-coming-to-smartphones-in-2018

All GPS devices have error, don’t base your fitness on them, use known track times.

GPS struggles with turns. Lots of turns on a track.