Distance measurement dilemma with Stryd

This is a post trying to figure out if there is an issue with Stryd, Polar or Garmin when the watches are configured to use the Stryd footpod as a source of data (distance, pace, running power) – as Polar and Garmin apparently report different distances run after the same run with the same calibration parameters.

Based on the observations from several days, it looks like Polar is using the calibration factor right as the distance measured by the Vantage V2 is the Stryd measured distance multiplied by the calibration factor [1.013]. Garmin has a lower value, albeit similar between runs – 1.006 for the most of days, 1.0076 for the third day.

I eventually performed a test with both devices calibrated with 100% and no auto-calibration, Polar’s distance was Stryd’s “raw” value as expected, Garmin still likes to use a 0.6% correction measuring less for some reason (I still need to understand where the problem comes from).

DayPolar VV2 DistanceGarmin F7X Distance Stryd Distance Notes
D1 (19.04)9.62km9.56km9.50km (*)F7X FW 7.35
Calibration factor 101.3% (both)
D2 (20.04)8.21km8.16km8.11km (*)F7X FW 7.35
Calibration factor 101.3% (both)
D3 (21.04)7.98km7.95km7.89 km(*)F7X FW 8.15beta
Calibration factor 101.3% (both)
D4 (22.04)6.25km6.22km6.18km (*)F7X FW 8.15beta
Calibration factor 101.3% (both)
D5 (23.04)9.40km9.36km9.30km (*)F7X FW 8.15beta
Calibration factor 101.3% (both)
D6 (24.04)8.34km8.30km8.26km (*)F7X FW 8.15beta
Calibration factor 101.3% (both)
D7 (27.04)11.62km11.56km11.49% (*)F7X FW 8.15beta
Calibration factor 101.3% (both)
D8 (28.04)7.41km7.36km7.41km (*)F7X FW 8.15beta
Calibration factor 100.0% (both)
Measurements over days (I used the same calibration factor for both watches, no auto-calibration)

(*) – Stryd distance was extracted by manually trimming the offline synced workout so it’s approximative.

Update: Day 8 [Final]

Out of curiosity, as I didn’t get a resolution from Stryd yet, I tried a different setup and changed the settings for the Stryd footpod for both watches today – I changed their calibration factor to 1.000 (or 100%) – so no calibration (or auto-calibration) – the expected result would be that all measurements will be identical.

The reality? Hmmm .. I just confirmed that something in the way my Fenix 7XSS is working with my Stryd is wrong. I don’t know yet is it’s Stryd or Garmin, if it’s something with my particular devices – I’ll raise this question on Garmin’s forums as well.

Polar measured 7.41km (with calibration factor 1.000)
Stryd’s “raw” distance is 7.41km (trimmed workout after offline sync with my mobile)
Garmin measured 7.36km (with calibration factor 100%)

So basically, Garmin does the magic in combination with Stryd and with the calibration factor of 100% is measuring less than Stryd/Polar as if it would use an calibration factor of 0.993. It’s like my Fenix has a strong preference of measuring with 0.6% less than the calibrated Stryd distance (in this case the calibration factor being 100%). This needs to be investigated and corrected if it doesn’t happen only with my Fenix.

Update: Day 7

I couldn’t resist and without the intention to continue the comparative tests, I have worn both devices for a session – I didn’t pay attention to the results, but they are comparable with ones in the previous days.

Garmin measured 11.56km
Polar measured 11.62km
Stryd measured a “raw” distance of 11.49km

So, nothing new .. Polar distance suggests a calibration factor of 101.3% whereas Garmin uses something like 100.6%.

Update: Day 6

The setup is identical to the previous day one, Polar Verity Sense on my right upper arm, this time with the sensor facing more the insight of the arm – this setup produced good results for the watch on the opposed arm as well.

Polar measured 8.34km
Garmin measured 8.30km
Stryd trimmed workout – 8.26km

Based on this side-by-side measurements, the results seem to be consistent and the behaviour is reproducible – both Polar and Garmin are using a correction factor with the “raw” distance value coming from Stryd, but based on the values I see it looks like only Polar is close to the defined calibration factor, whereas Garmin is using a smaller value, still a bigger than 1 value. Stryd support team has this issue on their table, as soon as I have a resolution or more details, I’ll add this in here – maybe I’m doing something wrong, maybe the settings are not exactly right or maybe this is a bug that needs to be fixed.

In practical terms, the 0.005 factor difference is not that impactful – for a marathon distance the effect of this difference would be 210m, which is close to negligible in the grand scheme of things – I can train as usual with Garmin Fenix 7X without any fear that I’m doing it wrong, but I still want to see this solved somehow.

Update: Day 5

Only one change today, same location, but I moved my Polar Verity Sense on my right upper arm to be closer to the Garmin, since I was kind of blaming the Fenix for not being able to keep the connection with the HR sensor for the full workout.

The results were in line with what I’ve seen in the previous days. Polar seems to be using the calibration factor, or close to it .. 1.011 vs 1.013 (official), Garmin “the usual” 1.006.

Polar registered 9.40km
Garmin registered 9.36km
Stryd registered 9.30km (the trimmed actual workout)

What I noticed though is the fact that Polar Vantage V2 lost eventually the connection to the HR sensor twice in the last part of the workout, whereas Garmin was just fine. So it might be that I need to accept that the HR sensor can only be properly used when on the same arm with the watch, otherwise you may have surprises.

Garmin didn’t gave any issues with the Verity Sense (being on the same arm)
Polar connection with Verity Sense had two drops in the last km of the run

Update: Day 4

Without any other changes from the previous day, except the running location, which may impact eventually the GNSS satellites connection, I had a shorter run wearing the devices in the same setup as day 3 (Vantage V2 on my left arm, Fenix 7X on my right, Polar Verity Sense on my left upper arm). The results are close to what I’ve seen in the first days: Polar displayed after the run 6.25km, Garmin displayed 6.22km and the trimmed version of the offline synced Stryd workout looks like 6.18km. That would suggest that Polar is close to the calibration factor with 1.0113 and Garmin had 1.0064. So far, the values seem to be consistent – Polar is close to the calibration factor adjusted distance, whereas Garmin is lower for some reason .. but at least is doing things the same. Surprisingly, Garmin and Polar hit the first kilometer mark at the same time, the sound from Fenix 7X was actually first, but in time that changed and Garmin marked the km laps after Polar did.

Polar VV2 tracked 6.25km
Garmin measured 6.22km
Stryd trimmed workout suggests 6.18km

As in the previous day, Fenix lost the connection at some moment with the HR sensor, being on the other arm .. but that’s still strange, as they were not that far away.

Garmin had some connectivity issues with the Verity Sense (I didn’t have any issues when worn on the same arm)
Vantage V2 had a good connection with the Verity Sense (not surprisingly, they were inches apart)

Update: Day 3

I operated some changes (too many?) to understand if there is a way to make the situation better with Stryd and Garmin – my suspicion being that there is a weak link there. So, I upgraded Fenix F7SS firmware from 7.35 to 8.15beta yesterday, I rebooted the watch as instructed and also checked early in the morning that my configuration of Stryd with the Fenix is as footpod and not power meter, following the guidance available here (actually I’ve been asked about the configuration by Stryd’s support representatives). What I’ve done was a little bit more even, I removed the Stryd from the accessories and re-added it as foot pod, with always distance/pace and auto-calibration disabled but with the same calibration factor 1.013.

The results? The distance was closer to Polar’s distance, but still not identical. Polar measured 7.98km, Garmin measured 7.95km and the trimmed Stryd offline synced workout measured 7.89km. That says that Polar is closer to 7.89 x 1.013 = 7.99km whereas Garmin is somehow in between with the 7.95km (that would mean a factor of approx 1.0076).

Polar says 7.98km
Garmin says 7.95km
Stryd says 7.89km

Other observations: The 8.15 firmware for Fenix 7 brings some new views (pace for example) for the workouts review, they look very nice.

Polar has real issues with the altimeter, the values variation is so high with no reason. Wearing it on my left hand had a positive impact, now it was closer to Garmin’s values, but during the previous runs (the track was very similar between days) it detected variations or more than 100m.

I used for HR monitoring the Polar Verity Sense on my upper left arm and I switched the watches, wearing the Polar on my left and Garmin on my right. For some reason (I believe communication problems, but I can’t say for sure), after the first 25 minutes, the Garmin registered the HR info in a very strange way, whereas Polar managed to get the data right (it was closer obviously).

Garmin had a communication issue with the Verity Sense after min 29
Vantage V2 maintained the connection with the HR sensor during the run

Update: Day 2

After yesterday, since the situation I think happened in the past, I decided to try to check the behaviour I’ve seen in several days so I went outside today again with the same setup (with only one change – I used my Polar H10 instead of Verity Sense – this wasn’t a great idea because the moisture was not enough and the strap values were garbage in the first 6 minutes of the workout) – using my Fenix 7XSS on the left hand and my Vantage V2 on my right hand connected both to my Stryd footpod with the same calibration factor 1.013.

The results?

Polar measured 8.21km
Garmin considered the distance to be 8.16km
Stryd trimmed workout (for the real duration of the run) shows 8.11km

Looking at the data, it looks consistent with yesterday’s observations as 8.11 x 1.013 = 8.2154km so that would suggest that Polar does the job right using the adjustment coeficient of 101.3%.

Tomorrow I’ll use the Stryd iOS app to measure the distance in parallel with the watches so that we won’t need the trim of the data recorded in Stryd footpod and synced offline with the app after the run.

Initial: Day 1

Today was one of those days when I was a weirdo wearing two sport watches at the same time … yeah, I know that regular reviewers are doing this all (most of) the time, but this is not my usual situation (at least it’s been a while since I did it last time). I was wearing both my Fenix 7X and Vantage V2 at the same time over night to check the sleep analysis on both devices – the trigger being the unusual behaviour of my Fenix 7X (FW 7.35) which was tempted every morning to extend my sleep with some tens of minutes if I was not moving vigurously and had a stable lower HR, even replaced my night sleep with some random period during the day when I was staying at my desk.

Anyway, that’s not why I’m writing this post – I still expect Garmin to release the next official 8.XX firmware to fix the sleep detection on the Fenix 7X – the reason is the way Polar, Garmin and Stryd are reporting distance for the same outdoor run.

I’m using my Stryd (the wind version) with a calibration factor of 1.013 on both watches, so the expectation would be that they will display the same distance after my run. Weirdly, that doesn’t happen and it’s not the first time I see that – Garmin (the 7X but the 6X had the same behaviour) is showing a shorter distance than Polar, even though both are connected to the same foodpod during the same run for the same period of time. Yes, I can’t start (or stop) the run at exactly the same time, but with approximatively 1 second difference because I’m wearing the watches on each arm and I couldn’t figure out a solution to start them at the same time (didn’t bother also, to be honest), but I think this doesn’t make any difference for the distance measurement.

After I finished the run, I collected the data from Garmin Connect, Polar Flow and Stryd Power Center (after an offline sync) and the result is below (the HR values are the same, since those were provided by the same Polar Verity Sense connected to both watches):

Polar measured 9.62km as distance for 0:54:47
Garmin thinks I run 9.56km in 54 mins and 50 seconds
Stryd measured 9.59km (with some movement before and after the run) – I need to strip the recording for the actual run part
When striping only the run (0:54:50) – Stryd says 9.50km

Stryd footpod measures implicitly the activity since you start moving the footpod and also don’t end the recording when you hit the button on your watch, but also continues to record until you stop moving – therefore you end up with some period of low movement before and after the actual workout unless you start it and end it at your door – that’s not my case, I’m usually walking about 40-50 meters before starting the workout. If you strip the beginning and end of the activity, you can “extract” the workout period. Maybe there are some tools to do it more accurate than I did it, but I’d say the result is satisfying.

In terms of calibration, both watches are using the 1.013 calibration factor and have the auto-calibration disabled, so they should be consistent.

Now .. my logic says that Stryd registered the “raw” information – so it’s 9.50km.

With a factor of 1.013 you’d expect both devices to report at the end of the same activity 9.5km x 1.013 = 9.6235 km. This is the value Polar reports, but not the value Garmin shows – I need to ask maybe the Stryd team to understand if there is any difference they are aware of.

Just to add more data (but not clarity), if you’d ask Strava what is the “correct” distance for the workout, you’d end up with new values:

“Correction” from Strava over the GPS track from Garmin FENIX 7X
“Correction” from Strava over the GPS track from Vantage V2

Garmin is right for sure about the elevation, that’s clear for me .. Vantage V2 just struggled with this and added 6-8 “artificial hills and descents”, that’s a consistent behaviour for my Vantage V2 which constantly had issues with the barometric altitude.

Fun fact: ignore the Wave Rider mileage, I’m not using those shoes anymore but I’m lazy at retiring the gear from Strava.

The GPS track is not that different, most of the time it shows I wore the devices on different arms and that’s all ..

The difference between 9.62km and 9.56km you might say it’s not significant and you’d be right – it’s only a difference of 0.62% so in the grand scheme of training for a marathon it doesn’t count … but since I was expecting the data coming from a single device (over Ant+ and Bluetooth) to be the same, that bothers me .. and I need to understand why.

Maybe that’s why I’m not progressing so well with my training…do I care too much about data ? 🙂

Fenix 7X FW 7.35 [FW 8.15b since the 3rd run]
Stryd Wind FW 2.1.16
Vantage V2 FW 3.1.1

Author: Liviu Nastasa

Passionate about software development, sociology, running...definitely a geek.

8 thoughts

  1. I sold my Stryd. It’s not only inkonsistent between devices. It’s even more inkonsistent when I am changing my shoes. Too much trouble. Gps is accurate enough. Most of the time much more than Stryd ever was.

    1. For me Stryd was consistent (right or wrong) and even now it seems that the issue with Garmin has the same manifestation. I haven’t checked until now much how the same data is “received” and displayed by multiple clients (watches), most of the time people just try to improve the accuracy of the measurements and don’t focus on the “data transmission”. Unless I’ll see some other weirdness, I’ll continue to use Stryd as a footpod at least because the distance/pace was more stable than what I’ve seen with Polar/Garmin watches (I haven’t checked how Garmin Fenix 7X would behave without Stryd in terms of distance and pace – the GNSS looks very promising).

    2. I did a test today using my Fenix 7X (best multi band/dual frequency setting) not receiving the distance/pace from my Stryd (wind version) and my Polar Vantage V2 connected to the Stryd (pace, distance, cadence and power from Stryd). The distance is different – not surprise here – Garmin had 8.30km, Polar (Stryd uncalibrated) had 8.25km, but the pace behaviour was better with Polar. I had 9 intervals of 2 minute to run at 5:05min/km and looking at both watches during the intervals I noticed that Stryd reacts quicker and is more responsive (that’s understandable considering the approach the foot pod uses) – Garmin was ok, but slower in picking up the pace at the start of the interval and at the end of it. Any fast change takes some seconds for the GPS driven watch to accommodate, so I’d say that my test confirms what most people say about footpods – they are consistent, not perfect and if you want a responsive tool, the footpod might do the trick for you.

      I understand that Stryd didn’t work for you – I had two devices (one non-wind and one newer wind version), they worked ok for me – yes, I still like to complain about things like calibration, their subscription policy – but they were consistent in their behaviour (good and bad). Considering the “math” they are using to get the data from the sensors, it makes some sense to have a bit of a difference when moving from one shoe to another or when changing the position even on the same shoe – just look at RunScribe (https://runscribe.com/) – they use their footpods to dive into the specifics of different shoes for example.

      I’m glad you find GPS accuracy with the new devices good enough to support your training efforts, it’s definitely easier not to worry about one more device/charger and even platform when analysing and improving your fitness.

  2. Hi Liviu,
    thank you for your accurate analysis… Just one question…

    Today after the first calibration (2000 mt on track) I set a new calibration factor = 102.00 on my Garmin… then I started a new session running for 25 laps on track and on my display of FR 955 the measured distance was 10100 mt. (while real distance is 10000).
    If I have a look at the stryd file after sync (in which I can find the whole session into one file… warm up, workout and cold down) the measured distance is 9910.

    So in order to calculate the new calibration factor which formula do I have to apply, 1 or 2 ?

    1) (10000*102.00)/10100 (using the distance displayed on the garmin)
    2) (10000*102.00)/9910 (using the distance measured on the stryd file after sync)

    thank you

    1. The way the calibration factor works is Displayed Distance (on FR955) = Stryd Distance x Cal Factor; In this case it was 9910 x 102%=10108 (I mean approx 10100). Therefor, the new calibration factor should be calculated using the real distance and the Stryd distance CF=10000/9910=1.009=100.9%.

      That means that if you’d redo the exact same test and you’d measure 9910m on Stryd, with this calibration factor you’d have been seen on your FR955 10000m.

  3. thank you so much!
    Are you agree if I say that calibration factor depends on the pace.
    I mean if I run at 3’50″/km for 25 laps on track calibration factor is different than if I run at 5’00″/km always for 25 laps on track? This in order to display same distance on garmin (10000)

    1. In my opinion, the calibration factor should stay the same regardless of the pace and let Stryd deal with the measured distance in a way that’s controlled by their sensor array and algorithms. The distance I run is the same, regardless of my pace – 1km is 1km regardless of my pace – even though Stryd doesn’t have the same accuracy when walking as with running (probably with walking/hiking we are hitting one of the limits of the algorithms/sensors, I assume).

      Eventually, we will (should) strive for consistency and not perfection – I mean, I made peace with the thought that even if Stryd is not perfect in measuring the distance, it’s more consistent (even makes the same mistake in the same way) with both distance and pace than any sport watch on the market (Garmin, Polar and others will measure distance and display pace less consistent than Stryd can measure). The calibration factor only tries to accommodate some of the differences between our style of running and the defaults of Stryd when doing their calculations.

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