gps fails to capture true speed – just weird

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  • #63948

    Today I did a Z2 hike on the steepest local terrain I have – Mt Defiance trail – which has an average grade of 22% and climbs up a series of mostly short switchbacks up the nose of a ridge. The local grade is often steeper. And I ran into this weird problem… the speed tracked by my watch would alternate between zero and more than three mph, which wasn’t what was happening on the ground; I never stopped, maintained a fairly steady, if slow, pace and my heart rate stayed within a narrow band for the entire climb (happy about that!).

    I am guessing that the GPS error was roughly the same size as many of the switchbacks I was on and so my watch wasn’t able to accurately track my speed. Not a big deal but not something I have run into before. Anyone else run into this or offer a better explanation?

  • Moderator
    MarkPostle on #63977

    Bill, I have had the odd workout where the GPS is all over the place. For mine (Garmin) it seems as though the GPS never really locked on initially and then was struggling the entire time for some reason. I try and give it a couple of minutes with a clear view of the sky to acquire the signal and number of satellites before moving. I put mine on the roof of the car while I am shoeing up etc, if absent minded this can lead to tracking the parked cars’ workout instead of yours. It would be interesting for you to try the same course again and see if that’s the problem or maybe it’s tree cover and/or switchbacks that are causing the issue.

    bill on #64025

    Thanks Mark. I will see if it is a GPS locking problem next time out. I will also see how the phone GPS tracks my progress on the same trail.

    MarkPostle on #64035

    Took a peek at the data from that workout that you mentioned, interestingly the vertical gain loss data looks quite accurate. This data may come from the barometric altimeter though or possibly even a map overlay but that would require accurate GPS data from the trail

    bill on #64043

    I know. If you look at the speed data it looks like I am alternating between stopping and moving fast (the speed varies between zero and three or four mph) but I was moving at a steady pace but for three or four tree falls I had to work around. But the elevation and distance data is pretty good, which is why I think the problem is GPS drift. I’ve noticed this in the garmin devices before… the point GPS can be off ten or fifteen feet.

    No matter. The data is good enough to figure out my climb rate, etc and gives me a benchmark for future training hikes on this hill.

    Thanks for the help, Mark.

    rcj on #64064

    Bill, if a second attempt at it results in the same thing, I wonder if it could be a setting on the watch. For instance, is it possible that a Run has a harder time picking up slower pacing, so it looks stopped, then jumps to catch up. While maybe a Walk setting could adjust the parameters of what the watch is looking for from the GPS? I occasionally see what looks like a non-moving period in slower climbing workouts where I know I was actually moving, but the overall distance tends to be correct. It makes me wonder about the interval between data points and that maybe I’m tricking the watch into thinking I haven’t been moving because I’m not far enough away from the previous point, which then watches up and overcompensates.

    bill on #64422

    rcj – that’s an interesting thought. i also figured that i should check to see how often gps is updating. i think the fenix 6 has a power save mode that slows down gps updates. thanks for the idea.

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