Why Strava distance is always 0.01 mile shorter than Garmin distance?

In slightly oversimplified terms Garmin sends a series of gps coordinates that maps your track. The issue is that the way Garmin adds up the distance between those points is different from how Strava calculates and adds up the distance between the points.

Yeah, the true half marathon distance is 21.0975 km. You were probably just a few meters short. I think Strava's stance about this issue doesn't show them in a good way. I remember seeing a comment from Strava that all GPS devices always measure distance long, which is clearly not the way with modern GPS devices if you read Garmin forums. The Sony chipset is especially bad in that regard. Distance measurement does no longer come directly from GPS. It comes from fusing data between the GPS and the accelerometer, and in my experience Garmin's distances often ends up being a bit on the short side when running on open terrain and even more short when running on trails. 

I think the right solution would be (a) to round exactly the same way as Garmin and (b) award a PB even when the distance is a bit short. How much, I don't know but something like 99.5% of the distance sounds like the right threshold. That would significantly reduce disappointment of users when they don't get a well deserved PB due to being a few meters short. 

Can we not just have a manual override button, where we can change the distance?  So for a 10k race that comes up short at 9.9 we can have it recorded as 10k, and that then gets factored into your average pace, etc. Main thing folks want is to see the race distance recognised in the app and rewarded as such, showing under my best 10ks etc. You already have a distance correction option that uses a different calculation so have another one that lets us round up or even round down for that matter? Maybe a certain percent of tweak.

This is a pathetic stance. I would suggest a vote amongst paying customers on this issue. Garmin didn't get where they are by fudging things. Grow a pair and admit you got it wrong Strava.

Having just ran a local training 10k which Strava shows as 9.99k, I just pulled open the fit file to see what was recorded. It shows 10,000.52m, so given what you say about rounding, can you please explain why this is the case? I'm happy to supply the fit file if you would like to take a closer look.

Hey @tombell, 
We're going to send this over to our support team so they can take a look at your account and find out what's going on. 

@tombell, I suspect what Strava does is that it subtracts the distance in the first sample from the distance in the last sample. If the first recorded sample wasn't at zero meters, for example if there wasn't yet a reliable GPS lock, then the resulting Strava distance would be shortened. That explains complains of some Apple watch users that they get a significantly shorter distance on Strava. I also recall seeing a similar issue with a Suunto watch. 

Also, Strava support article https://support.strava.com/hc/en-us/articles/216919487-How-Distance-is-Calculated mentions that the data is parsed into a stream, which further supports my suspicion. The article doesn't say that Strava looks at the FIT file header that would have the correct device reported distance. Instead it specifically mentions the stream of (I assume) samples stored in the file. Take a look at your FIT file. What distance do you see in the first sample? 

Why is it that on my peloton, the run completes at 5.00km but then sometimes it appears in Strava as 4.99 while other times it shoes as 5.00? I’m missing every second run in my totals? 

If you could help me! I’m almost 65 and using running, and mountain running, as my physical and mental health tool. Using my Garmin watch and Strava I just registered a 5:51 mile. What does that translate into minutes and seconds? Thank you!!

> there is research supporting that in many cases, GPS tends to overestimate true distance

Here is a research from 2020 that makes an opposite conclusion - that nearly all modern GPS devices underestimate the distance:
https://mhealth.jmir.org/2020/6/e17118/#ref1

Specifically this:
Please note that this research has focused on running distance measurement. One of the conclusions was that devices tend to measure distance short when running as opposed to walking or cycling activities that tend to be more accurate. 
So, Strava by always rounding the distance down adds insult to the injury.

Hi @Silentvoyager - thanks for sending that paper by. Admittedly a lot of the research I was referring to is a bit older than the 2020 one (except for the International Journal of Sports Science & Coaching one, which I believe is also from 2020), and the variance in sample sizes and terrain that they were tested on is pretty high (i.e. tree cover vs. buildings vs. straight paths vs. tracks). I've linked some of those below (most of them are simpler articles and have the actual studies cited at the end)

• https://www.trainingpeaks.com/blog/how-accurate-is-your-gps-watch/
• https://runnersconnect.net/reliable-gps-watches-running/
• https://www.dcrainmaker.com/2010/11/sport-device-gps-accuracy-in-depth-part.html

This may have changed and we are open to talking with some device manufacturers to see if they have any guidance or more rigorous results, especially with the fusion of accelerometer/pedometer-based data to the distance. Transparently, the only studies we've done recently tend to involve race courses which we've found are almost always recorded longer on devices than the certified distance, but this may be a bias of looking at races where it's hard to exactly follow the tangents, and extra distance is also accumulated that way.

Hi @varun, thank you for the continued discussion on this topic!

You are correct that in the past GPS measurements had a tendency to be too long due to the wobbling effect, especially at slower speeds where the distance covered in one second (which is the typical sampling rate) is comparable to a typical GPS error rate.

Since then two things have happened.

1) Most modern devices no longer use GPS alone to calculate the distance. They perform so called sensor fusion where the data from gyroscope, digital compass, and accelerometer is considered. There is a heavy error correction in which devices try to predict what the next position should be based on your movement pattern and input from those sensors. That not only eliminates the wobbling but also straightens the path into turns. That's why it has a tendency to shorten the distance. Furthermore, some devices seem to favor input from sensors like accelerometer as the primary source of distance and use GPS to continuously calibrate it. There is a lot of evidence that Garmin does that for walking and running activities. Because of that the path recorded from GPS no longer matches the recorded distance and that's why Strava distance correction almost always changes the distance (usually increases the distance).

2) The second more recent development is that a lot of modern devices now have multi-band (multi-frequency) GNSS, and that drastically improves accuracy. The accuracy is so great now that I can often see whether I was running on a sidewalk or on a road next to it. That alone eliminates the wobbling effect to a minimum. In my experience the running distance still tends to be on the short side, but to a lesser degree than with the generation of devices mentioned in that 2020 article I linked above. 

This is also the same on Zwift, you always end up with .01 less distance!! Not just Garmin.

Strava does the right thing, Garmin gets it wrong. Distances should always be rounded down, times always rounded up. That's how it's done at races per the rules of World Athletics.

The official marathon distance is 42.195 km; the official distance in miles doesn't exist since converting 42.195k into miles gives you an infinite number of digits behind the dot. Running even a fraction of a nanometer less means it's not a marathon and any record or Boston Qualifier won't count.

Per the rules of World Athletics, a marathon is 42.195 km, not 42.19499999999999999999... . If the distance covered is even a fraction of a nanometer short, your sales manager won't get a bonus from World Athletics. That means no world record, no olympic ticket, no Boston Qualifier... .

Garmin should round distances down like Strava does.

That's not how World Atletics and any national athletic association does it. Official race rules say to round all distances down and all times up.

Always rounding ...5 up means you round up more often than down. That's why most scientific journals require their authors to round ...5 to the nearest even number to prevent a systemic error.

On the other hand, World Athletics requires all distances to be rounded down because official race distances are minimum distances. Similarly, all measured times get rounded up.

The reasoning being that running a marathon in 3:00:00 means you ran a course of at least 42.195k (and not a fraction of a nanometer less, as measured by a Jones Counter), and it took you 3 hours at most and not a nanosecond more (as recorded by the electronic timing system).

Great answer, except for one thing. The official marathon distance is not 26 miles and 385 yards. The one and only official marathon distance recognized by World Athletics is 42.195 km as set using the metric system for the 1908 Olympics in London. We can blame Queen Victoria for that.

The official marathon distance in miles is 42.195/1.609344. This is a number with an infinite number of digits after the dot, so the official marathon distance in miles does not exist in decimal notation.

Not a big deal, but Strava always shows avg speed a tenth or two lower than my Garmin. On some rides it takes a bit of effort to get those 10ths! Why can’t Strava just accept the exact numbers from Garmin device without manipulating them?!! Is Strave the ‘ride police’ to enforce their interpretation of a ride that Garmin recorded? 

> Per the rules of World Athletics, a marathon is 42.195 km, not 42.19499999999999999999...
The main issue is that Strava doesn't really know if that was 42.195 km or 42.19499999999999999999. The same actual distance could be measured by a GPS device as 41.77 km or as 42.62km - that all would still be within a very respectable 1% distance accuracy depending on whether there were some buildings, trees, or any other obstacles. Many GPS devices do much worse, especially when running on trails.
Arguing about which way to round when a typical GPS error is far greater is just stupid. Everyone refers to that as "Strava tax"! 

I just had this exact problem. Did a 10K training plan, and the watch automatically stopped the activity at 10.00km. So now Strava doesn't count my 10km PB because it's only 9.99km.

There is also no way to edit it on strava to fix the distance. I guess I would have to edit the GPX file and upload that.

When I did other runs I made sure I went a little bit futher just to make sure it actually counted the distance but that's not possible with the Garmin training plan runs.

edit: I fixed it by exporting as a TCX from Garmin, then editing the last Trackpoint in the file to be a couple of meters along the road by checking the lat and long coordinates on Google maps. I then imported the edited TCX and it still showed as 9.99km but after using the "Correct Distance" option in Strava it updated to 10.00.

The original recorded activity was actually 10000.1 meters according to the TCX, so it looks like Garmin did record a little extra to try and work around this problem - but it didn't work.

It’s my understanding that this is also impacted by what Garmin considers moving v. Stopped being different than how Strava views it- not sure how it could possibly be different, but apparently it is.

would be great to see some sort of universal standard here.

It's not just Garmin; I have friends who run with COROS and Apple watches and suffer the same "tax" when activities are uploaded to STRAVA.

I understand your frustration, but I actually think Garmin should be the one updating how they round numbers.

Cause you didn't run a half marathon, you were short by 5 meters haha.

I prefer the rounding down approach.

You don’t know if you were actually 5 meters short or not. That’s what your GPS device has measured, but as we know modern GPS devices tend to measure running distance short - see the study I mentioned earlier in the tread (https://mhealth.jmir.org/2020/6/e17118/#ref1).

Why there are so many cases when someone finishes an officially certified race, but the distance comes up a bit short, and then they don’t get the achievement?