Fenix 6 Pro FIT vs GPX Files - Distance Discrepancy

I have seen many threads addressing the Garmin Fenix 6 Pro (and other Fenix 6 models) and the inaccuracy (specifically being short) due to smoothing done by algorithms for the FIT files.  For many months, I have had to use the "Correct Distance" feature in Strava after my run is synced due to the Fenix 6 Pro FIT file inaccuracy.  However, today I saw that you can test the issue  by exporting the GPX file from Garmin Connect and re-import it to Garmin Connect.  Lo and behold, all my missing distance showed up.  Today's small run of 6.3km actually showed 6.3km vs the 6.0 reported by the FIT file.  I also traced the route out on Google Maps and Strava "Create a Route" and got 6.3km.

My questions:

  1. Are there any fixes to address the FIT smoothing problem for the Fenix 6?  
  2. Is Garmin tech support doing anything to address this problem?

I've been a long time Garmin customer since the Forerunner 305 days.  The Fenix 6 has been, hands down, the worst GPS experience.  Yet, now I see that the watch itself is actually recoding the distance correctly but the software algorithm smoothing is the true culprit.  I just wish something would be done.

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  • It is not the FIT smoothing problem.

    The root of the problem is that device distance is not consistent with the track. A FIT file has both the data from device and the track. A GPX file has only the track. When you export a GPX file you basically ignore all the summary metrics produced by the device and get the raw data only. Then as you import that GPX back, since the distance is unknown, Garmin Connect has to calculate it again by adding distances between all consecutive points in the GPX. But the points themselves are exactly the same as in the FIT file.

    Furthermore, when a run is synced to Strava, it uses the device data stored in the FIT file header rather than the raw data from the track. Therefore Strava inherits the same data as displayed in Garmin Connect. However when you do distance correction in Strava, it ignores the distance from FIT file header and calculates it from the points on the track using the same method as above - by summing distances between the consecutive points.

    So in short, the root of the problem is that the data produced by Garmin isn't consistent to start with. I too think that in 99% of cases the raw distance along the points on the track is closer to accurate distance than the one displayed on device and in Garmin Connect. The only exceptions would be the following two cases:

    • When a track is extremely wobbly, for example when running in a city downtown among tall buildings or when moving very slowly on a steep mountainous terrain
    • When smart recording is enabled - distance from the track may end up being shorter in this case.

    And regarding the reason why Garmin isn't willing to change this, I think that Fenix, not being a sport watch but rather an outdoor recreation watch, is tuned for slower activities like walking and hiking. I've noticed that at slower speeds GPS track becomes increasingly more wobbly and may end up being too long. I've noticed that my slower activities such as ultra distance runs have more accurate distance than faster paced short runs. At faster speeds there is less wobbling but more corner cutting. 

  • Thanks so much for that detailed response.  It really helps me understand it further.

  • This is, i think, the problem with open water swim activity, it understimates distances a lot resulting in slow pace.

  • It's not just that. Here's a track comparing the recorded speed, vs the algorythm used by our own @AndersB. This is from a run where I was doing pace repeats. The pace recorded by my phone is similar to the AndersB data. This is in a town, wide roads, no trees, no high buildings.

  • As well as the difference in distance it is quite eye opening to look at the fit vs gpx tracks overlaid with pace.

    Here is a comparison of 6X pace data from a walk, with the fit pace on the left and gpx pace on the right. The fit pace is quite smoothed, or perhaps more truthful, whereas the gpx data shows my pace to be constantly all over the place.

    (BTW, fit distance was 4.77km while the gpx distance was exactly 5.00km.)

  • I've been a long time Garmin customer since the Forerunner 305 days.  The Fenix 6 has been, hands down, the worst GPS experience

    Could not agree more!

    So bad, I resorted to buying a stryd footpod so I now have accurate distance data to train with as with the Fenix 6 I was losing mileage ( a full mile over a half marathon) while the fenix 5 running next to me was spot on. Truly unnaceptable that this watch is that bad and more money is required to make it accurate. My FR235 was far better.

  • This is, i think, the problem with open water swim activity, it understimates distances a lot resulting in slow pace.

    Not necessarily true. As I've said on a number of occasions, I swim a marked 2km course. My watch records close to 2km. My pace is slow not because of the watch but because I am slow!

  • In the end, my biggest frustration is that I want accuracy.  If the GPS data is there and being recorded properly, I don't want the FIT file to be skewing the results.  I am ready to get a new device and sell my Fenix 6 Pro on eBay.  Like I mentioned, I've been using Garmin devices for over 10 years and don't really want to change platforms.  I am an ultrarunner (and occasionally cycle) and value long battery life.  If I knew the Fenix 7 wouldn't have this problem, I'd try it but now I'm gun shy on what to do.  Anecdotally, my Fenix 5 seemed to be better in terms of FIT files and accuracy but I haven't done a scientific approach to wearing both to compare distance recorded on same runs.

    On a side note, I've also had a quirky issue where my Fenix 6 Pro gets occasionally "wonky" for measuring my HR but only on my left wrist.  It would show my HR surge by 20 BPMs for now reason.  After some experimenting, I found that if I changed to my right wrist, it would immediately drop to what I perceived as the probable HR.  For that reason, I only wear it on my right wrist when running.  FYI, I did play with the setting for left/right wrist but it had no effect on the errant HR drift.

    Long story short, not happy with the F6Pro and I want a new Garmin that will work! :-)

  • And i have bring (and not only me) on a number of occasions proofs that MY activity are all "shorter" than in reality. We can go on like this indefinitely.

  • Yes, it is very disappointing. Fenix series has too weak GPS signal for Smoothing algorithm and this makes so bad. Very often track is not so bad, but the distance is too short (2-10%). I have already started warning people with hashtag #GraminFenix6GpsSucks to not buy this device if you want to run in the forest. Has everyone already highlighted this issue to Garmin? GPS/construction issue cannot be improved, but they should be able to do something with approximation algorithm as it seems to be even worse than GPS itself (there is no justification to reduce the distance so much).