Treadmill calibrated distance not sent to Strava

Yep, of course you're right. I'm confused by the technical answer though, even if Strava used the calibrated total distance, what about all the other data points? Surely they all need to have the same correction factor applied otherwise the final distance data point would be meaningless. The TomTom may be making the correction across all points which results in a single full set of usable data.

I'm just not as sure as some obviously are that this is not solvable by Garmin, without much effort... or even caused by Garmin not applying the calibration properly to the data.  As you say though, whatever the issue, this shouldn't be our problem.

5609075 said:

what about all the other data points? Surely they all need to have the same correction factor applied otherwise the final distance data point would be meaningless.

Strava will have no problems with it. They already do recalculate all track points of an activity, for example when you use the function "Correct Distance" at GPS activities, so they already have functions for that.

5609075 said:

I'm just not as sure as some obviously are that this is not solvable by Garmin, without much effort.

Again, why should Garmin fix something that works perfectly al right in Garmin Connect? Strava is not their business. They have enough of their own problems, they will not spent their precious resources on fixing problems of others. Strava has to fix it.

What problems? this one seems like an easy fix, perhaps the TomTom devs saw the use case that included Strava and alocated resources accordingly... or maybe it really wasn't a difficult fix.

Correcting distance for GPS activities only involves splitting, or removing sections from the start or finish if I remember correctly, anyone feel free to jump in. It doesn't apply a calibration, because, well... you wouldn't.

Thanks for your input chap.

5609075 said:

Correcting distance for GPS activities only involves splitting, or removing sections from the start or finish if I remember correctly,

No, during the correction, all distance and pace information of all track points in the FIT file is being stripped, and trigonometric distances between the individual track points are used instead. It is practically the same what is needed at the treadmill - ignoring the trackpoint pace and distance data, and calculating it anew. This time using the total_distance instead of the trigonometrical functions. 

See it explained in more details at How Distance is Calculated – Strava Support (especially the paragraph "GPS-based, Strava post-upload approach")

I think you are conflating two different functions.

Yes, all apps use trigonometry to calculate distance based on whatever data set is being used, that's basic. You were previously talking about Strava's additional function of being able to adjust distance of GPS activity though, which is what's relevant to this conversation, and which involves splitting or cropping, in other words NOT recalibrating incorrect data.

Why on earth would you want to calibrate incorrect GPS positions in a third-party app? if your watch or sat-nav is not showing correct positions, you're going to need to send it back to the manufacturer because it has a more serious fault than what we are discussing.

Anyway, for any type of uncalibrated data, recalibration needs to be done at the hardware end so that it learns and becomes more accurate. Same deal for any measuring instrument. Clearly this is something that the Garmin does because it becomes more accurate based on your adjusted total distance.

What it doesn't do is apply that adjustment to all the data before uploading, it just gives a single final adjusted distance and expects any third-party app to know what that particular field is and what to do with it.

Your initial argument was that Garmin has provided all the data and there's nothing wrong, it's all down to Strava when clearly it's a more nuanced argument than that. You also argue that we the customer are the ones that should be ensuring Strava fix their software and no communication is needed between Garmin and Strava, again, that's too simplistic imo. and leads to 5 years of no resolution.

5609075 said:

Yes, all apps use trigonometry to calculate distance based on whatever data set is being used, that's basic. You were previously talking about Strava's additional function of being able to adjust distance of GPS activity though, which is what's relevant to this conversation, and which involves splitting or cropping, in other words NOT recalibrating incorrect data

No, the function Correct Distance at Strava indeed re-calculates the GPS distances of each trackpoint, and mostly does not even make any cropping or splitting (unless there are clear outliers). You need to look at the FIT file format and understand how it is build - each track point contains not only the GPS position, and the physiological metrics, but also the pace, and the distance from the start - those values may come from the GPS, but very often they don't - not only when there are speed sensors, foot pods, or HRMs providing that metrics, but also when the GPS chip reports accuracy below safe threshold (bad conditions, obstacles, underpasses, dropouts, GPS drift, ..) the watch immediately switches into the indoor mode, and stores the pace and distance data calculated with the help of your calibrated pace for given discipline. So depending on the quality of the GPS signal, there may be a significant number of track-points using the calibrated metrics, instead of GPS-based pace and distance.

For this reason, whenever you use the function Correct Distance at Strava, the result will differ from the original (I mean especially the distance, and consequently also the pace), unless you have perfectly clean FIT file with only GPS metrics (I did not see one yet, in many years), or perfectly calibrated stride pace, and you run at the same speed and cadence as the calibration was done at. At cycling with a speed sensor, the data from the sensor will be more accurate than the GPS calculation anyway, so I would not use the distance correction on such activities.

Sorry but we're talking about two different things.

The GPS data that a watch uploads to Strava is handled using whatever mathematical and data processing techniques that are appropriate. Any discrepancies resulting from this process will be small and not relevant to the discussion.

What we're talking about though is recalibrating when there's a discrepancy between measured and actual distance travelled. In my case a 20% difference.

With Strava, all a user can do to edit a GPS run, once uploaded, is 'create segment, crop, split, flag or delete' that's it. If you GPS distances are wrong, your hardware is broken. When you initially mentioned Strava function to correct, you specifically mentioned 'Correct distance at GPS activity', which is what is provided to the user as I've described above. You're now talking about how Strava handles the initial data to calculate distance which is something different.

If a watch uploads data to a third-party app and one data point is out of the ordinary, it will most likely be flagged as erroneous and ignored, whether that be at the end, the start or somewhere in between. If the Garmin uploads a long string of data with just a single corrected distance at the end, it may be that this is the problem, and that data point is treated as erroneous because it's too different from all the proceeding points.

Whatever the reason, re-calibration, which is what we're discussing, should not happen in any app because there's no feedback from app to hardware and there shouldn't be.

We're simply talking about the data uploaded and the reason for this issue. Perhaps it's best to avoid talking about GPS data calculations as I think that's somewhat of a red herring.

5609075 said:

Sorry but we're talking about two different things.

Maybe. I am speaking about the function Correct Distance in Strava (on the web, not in the app!), that does exactly what I described, and what is also described in more details in the document from Strava that I linked previously. It recalculates all the trackpoints, using the GPS positions. And it then, of course, also recalculates all the pace data, so that the pace graph matches the new total distance - that's exactly the same functionality that can be used for the treadmill activities.

It does not modify the GPS track at all (unless there are evident outliers, which is not very common), but the resulting distance (and pace) will almost always differ from the values reported in the FIT file. In my case, at running it is typically some 10% difference, and 2-5% when running with the HRM-Pro that supplies a more accurate pace calibration (a new feature since a few weeks).