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Minimum Speed to Properly Accumulate Distance?

nord0296
over 4 years ago

My FR935 has poor GPS performance in Open Water Swimming (several threads over on that side of the forum), so in order to try to record better distance and path I dropped my Edge 530 in a floating buoy behind me today.  I turned off Auto Pause, pressed start before starting... swam and pressed stop.

To my surprise my Edge 530 recorded 1.25 miles while I swam ~3 miles (FR935 claims 3.45, but I think it a bit high).

Oddly enough the Edge file has a really good looking track (I think it's complete and accurate) and the "speed" field in the FIT file doesn't seem to show any gaps.

I know this isn't specifically a use case the team may have planned for, but it sure seems weird.  What happened?

FR935:

Edge 530:

DCRainmaker Analyzer comparison of speed (purple - Edge 530):

Here's the distance comparison... maybe Edge only adds to distance when the GPS signal is really good (yet somehow records speed)?

Interestingly if I strip out the "Distance" field using fitfiletools.com "Field Stripper" and then load to Strava, for example, it may recalculate correctly.

  • 0 over 4 years ago

    Edit: I'm dumb and clearly missed the part about auto-pause. I'm as stumped as you are. I'll leave my post below but I doubt it is relevant in this case.

    Thoughts from my side:

    • do you have Auto Pause turned on? This would prevent it from tracking distance under a certain speed, and is often enabled on bike computers to reduce the noise that can occur if a bike is going under a few km/h.
    • Do you have the recording interval set to Smart or Every Second? I don't think that would make a difference that is anywhere near what you are showing, but it can cause very minor track and distance differences. I set it to every second.
    • What satellite setting are you using? I have mine set to GPS + Galileo as I am in Europe and want to get data from multiple constellations. As you're recording on a lake it shouldn't be bad either way, but that can be worth checking.

    Those are at least things that come to mind from my side, with auto pause being the most likely culprit for that distance indicator.

  • 0 over 4 years ago in reply to sublimity

    I appreciate the reply, and don't worry about asking about Auto Pause.

    Just to be sure I just checked and I am set for:

    • Auto Pause Off
    • Recording (system settings applicable to all profiles, I assume) - 1s
    • GPS + Glonass is set for this profile

    I am using Beta 5.51 for what it's worth.

    It's not a huge issue per say, but I found it quite odd that I thought I would post it.  Clearly it is a unique use case that differs from what most (nearly all?) Edge users would try.

  • +1 over 4 years ago in reply to nord0296

    The device is designed for cycling so there are some assumptions about the minimum speed that would occur. These assumptions are used to filter out GPS position drift. If you are travelling too slowly, such as walking or swimming the unit may treat that as GPS position drift and will not accumulate distance, resulting in an under reporting of distance.

  • 0 over 4 years ago in reply to aweatherall

    Thanks for the reply and information, I glean the threshold is roughly 2 mph Slight smile

    If anyone wants to do this in the future you get good path data recorded and just need to strip the recorded distance field from the fit file (I.e. using fitfiletools web site)

  • 0 over 4 years ago

    This post just made me think of something off-topic that I am going to try next winter.

    I have a Fenix 6X and I noticed last year that when I am snowshoeing and breaking trail, I am frequently going <1.5 mph, it frequently will under-count my speed and distance in a similar way to what you saw.  I did a 5 mile out-and-back route breaking trail and coming back and on the way out it only counted 1 mile and the track while reasonable, showed many stop/jump/stop/jump/stop/jump in the speed fields.

    When I next go snowshoeing I will use the open-water swim setting just to see if that works better since swimming is slower than walking/running normally anyway.

    Probably will screw up my step count, but it will be worth an experiment.

    To make this mildly relevant to the Edge 530, before I got my Fenix, I used my old Garmin Edge 800 when backcountry skiing and snowshoeing and I never noticed the problem with it not tracking very slow movements.  I suspect this is a battery saving optimization that is too aggressive for "non standard" uses of the Edge (and Fenix for that matter.)