@Philreedshikes the -2 to +3 is incorporated in the total of 9' gained over the 6 readings. The "altitude gain" metric only counts situations where a new reading is higher than the previous, ignoring all the down segments. That's what results in the discrepancies.

I've tested this in mountain settings where I know the altitude at the bottom and the altitude at the top. The trail is all upwards with no "dips" along the way. My smartwatch as well as smartphone apps show an altitude gain that's as much as 50% more than the different between top and bottom. Even if there was the occasional dip or down section it wouldn't be able to account for that discrepancy.

Similarly with maximum speed. Say I'm averaging 3mph. Sometimes I'll see the maximum speed as a bizarre number, say 10mph. There's no way I can walk that fast. My inference is the GPS lost signal for a while and somehow screwed up the speed calculation.

Likewise with moving time. How do the systems calculate that? GPS position readings aren't perfectly accurate. If you sit down somewhere for a rest successive readings could show you a few feet apart, i.e. moving. Perhaps there's some compensation for slow movement that's intended to deal with that. If so I'd imagine each system has somewhat different criteria. So more discrepancies.

I take these readings with a large grain of salt. For me they're only good as an estimate of what I've accomplished in that hike or relative to other hikes.

Incidentally I go for a city walk every week with several other guys. We all have smartwatches. They're different makes and models. At the end of our walks we sometimes compare the distance we've gone. It's not unusual to see 20% differences.

 

...Wanderer


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