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Walk distance variance in different gps modes

6125703
over 1 year ago

I thought I understood my FR255 after a couple weeks of ownership, but now am more confused. I have been using it with slightly hilly walks in my neighborhood in the San Francisco Bay area. I've been repeating the same routes many times while trying out different watch settings. This is mostly open skies while passing some bushes and trees. I imagine the biggest obstacle between the watch and sky is my body or my walking partner's body.

Weeks ago, one favorite route measured out as 3.37-3.39 miles on multiple "Walk" activities I'd set to use the "auto" (SatIQ?) satellite mode. This distance matched my expectations though I've never tried to measure it with a different instrument.

More recently, I made a copy of the "Walk" activity and set the GPS mode to "off", to test with just pedometer estimates. It always estimated a shorter distance perhaps 75-85% of the expected value. I found this a little frustrating that it seems to ignore the customized walking stride length in my profile, so I resumed playing with GPS settings on this copied activity.

I repeated the walk twice with the UltraTrac mode enabled and the watch recorded 3.25 miles. The track on the Garmin Connect Mobile map is a little jagged, veering off the street or rounding a few corners, but it's hard for me to believe it really cut 4% from my actual path.

Yesterday, I did the walk with the "GPS only" mode and the watch only recorded 2.88 miles as the activity distance. However, when I look at it on the Garmin Connect Mobile map, it looks like it followed the route correctly with just a little more wobble than the original SatIQ tracks. I don't see any gap or truncation at the start or finish. I don't see how it can lose 15% of the distance while showing the same loop around the same streets. It looks like a better track (more faithful curve following) than the UltraTrac but measures as shorter!

For all these walks, it seems to record consistent moving time and average cadence info, with only minor variations that I think correspond to my actual pace being a bit off some days.

Does anyone know how a track could look like it follows the right course but then somehow lose a bunch of distance?

I should also mention that I have "3D distance" enabled for all these activities. I do see some variation in the total ascent/descent figures on different days but nothing dramatic enough to explain a half mile difference. One reference SatIQ walk showed 243 ft gained/230 ft lost. The mysteriously short gps-only walk showed 256 ft gained/236 ft lost.

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  • 0 over 1 year ago

    After resolving my "ephemeris missing" issue, another UltraTrac session around the same route reported 3.35 miles, the closest I have seen so far to the SatIQ distance. In spite of this, the actual track was a bit rough, cutting off several corners and ending a few hundred feet short of where I actually pressed the STOP button to end the walk.

    I don't really care about the track position accuracy for this kind of repeating walk, but I would like to have consistent distance/speed measures to go with the heartrate and altimeter info for the basic fitness tracking aspect.

  • 0 over 1 year ago

    Try Switch the 3D Distance and Pace to off, Many have noted in various forums that it creates inconsistencies 

    Why don't you just leave it on SatIQ , or is battery life an issue? I always use mine in Full Multi mode.

  • 0 over 1 year ago in reply to S.M.R.

    Ha, is there any feature that just works in either mode?  I don't like the idea non-3D distance because it makes no sense to me. I always think in terms of odometer distance and I gravitate to hills and mountains, not flat places.  But I'll try to remember to try it too. Maybe the errors introduced by the non-3D are smaller than all the inconsistencies?

    And yes, I am experimenting with my new watch to try to find my ideal settings. I am the kind of person who dislikes having to plan to charge things. I have ignored the smartwatch world until now because of my understanding of their absurdly short battery lives. I'm the kind of person who gets 7-10 days out of a smartphone charge and who previously had quartz watches that last "forever".

  • 0 over 1 year ago in reply to 6125703
    6125703 said:
    I don't like the idea non-3D distance because it makes no sense to me.

    But if it doesn’t work reliably? I think there is a reason, why 3D distance is not activated by default.

    For general accuracy of FR255: One of my favorite tracks for running is on a road through a forest, so some disturbance from the trees to be expected.
    Google Maps (and Open Streetmap too) say it is 10.3 km long.

    The FR255 with activity „Running“, using „SatIQ“ and having 3D distance „off“ (=default), gives me 10.32 km distance in 95% of recordings. In 5% it is a little longer or shorter, I have had values from 10.28 to 10.35 or so. All good, some minor differences can happen, if the circumstances are not optimal.

    Conclusion: The default settings are good and accurate. If you need the watch to save battery, you can use „Ultratrac“, otherwise SatIQ is reliable and a good compromise.

  • 0 over 1 year ago in reply to Grumpy from dawn till dusk

    Yes  I'll try testing it. I turned it on the first day I had the watch since it sounded like what I'd prefer.

    It didn't occur to me that it could cause problems like making a track measure shorter. In fact, I would have thought that it would never be shorter since altimeter error can only make a path appear longer in terms of geometry, right?

    So, in my mind, I was testing that out didn't inflate distances nor elevation gain/loss. I have seen how the gain/loss can get exaggerated on a phone based tracker that uses gps without considering barometric altimeter. So far the elevation profiles have looked really clean and consistent to me after my activities. The calibration might be off for absolute level, but the relative profile seems much better than gps would.

  • 0 over 1 year ago in reply to 6125703
    6125703 said:
    It didn't occur to me that it could cause problems like making a track measure shorter. In fact, I would have thought that it would never be shorter since altimeter error can only make a path appear longer in terms of geometry, right?

    Yes, I would agree with you. But it seems, the implementation of that is not that easy, at least seems not to be reliable at the moment.

    6125703 said:
    So far the elevation profiles have looked really clean and consistent to me after my activities. The calibration might be off for absolute level, but the relative profile seems much better than gps would.

    I agree. Before I had the Forerunner 245 without barometric altimeter, with FR255 altitude information seems to be smoother.