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

[Silentvoyager]

I know it is a small thing but it is one of those things that is quite annoying about Strava. You know that situation when you finish a run and notice that it is very close to a round number of miles, than you run a bit further to make it e. g. 5 miles. Then it uploads to Strava and shows as 4.99 miles in some parts of the app and 4.9 miles in other parts of the app (e. g. the training calendar). Can Strava stop doing that? 

[varun · Accepted solution]

Hi @Silentvoyager, 

We know this has been a long-time quirk, and often a frustration for many athletes, including some of our own employees! The reason this happens is that Garmin (both the device and Garmin Connect) rounds up distances, whereas Strava rounds down. Distances are always stored in meters on devices and then shown on Strava in the athlete's preferred units (i.e. miles/yards for imperial and kilometers/meters for metric). So for example, a distance that comes over in a file as 4995 meters will get displayed on Strava as 4.99 km and on Garmin as 5.00 km. Similarly, if on your watch you run until you see 1.00 miles, that might actually be something like 1608.0 meters (0.9994 miles) and Strava will show 0.99 miles.

It is also worth clarifying that most Garmins, even relatively older ones, provide a distance directly in the file that Strava uses. There are cases we calculate distance ourselves from the raw GPS points; for example, if a certain device doesn't provide it. And athletes can also choose to use our calculated distance to override the device one - see How Distance is Calculated for more. 

One other nuance for the question from @MattS_bsb regarding race distances like a half marathon or marathon. There are features like Best Efforts and Challenges based on those Best Efforts that require athletes to complete a certain distance. One thing we have seen is athletes who run 26.2 miles and not receiving credit. This stems from the same issue above - that a marathon is 26.22 miles when converted from its official distance (26 miles and 385 yards), and athletes are often slightly short.

We understand that this is a bit of a philosophical stance we've taken - the digital version of "running all the way to the line". There is a tradeoff here between the consistency with Garmin and satisfaction of the round numbers (because yes, we've all been there), and the care we aspire to take with data correctness and integrity. This is one of an exceedingly long list of nuances around data as we pursue making Strava work for thousands of devices, each with its recording subtleties and differences.

Hopefully that helps clear things up and explains why you might see some crafty athletes running til they see 1.01 on the watch. We will strive to continue using this forum as a place to transparently explain more of these types of situations to athletes.

[Silentvoyager]

I have just been discussing this exact issue on Reddit and saw the following comment:

I just looked at several of my runs and on a 10k distance, every one is short anywhere from .01mi to 0.03mi. My watch is uploading in FIT and has the distance embedded in the data. Are we sure it's just rounding? I personally don't care if it's off 0.03mi on a 10K distance.

I understand that the 0.01 mile difference can be explained by rounding, but how the larger differences would be explained? The same reddit thread gives another example of 0.06 mile difference when uploading from Apple watch. Now that I think about that I also recall seeing 0.02 mile difference when uploading from Suunto 9 Baro via FIT format. 

[MattS_bsb]

@varun  This isn't just an issue with Garmin, as I have friends/fellow runners who experience this with COROS watches.

Also, I would refer you to my post on this topic where I cited non-GPS activities such as indoor rowing where the source data is registered as one distance, but STRAVA arbitrarily rounds down, e.g. 5000 meters on a C2 rower calculated as 4997 on STRAVA.

https://communityhub.strava.com/t5/devices-and-connections/distance-discrepancy-between-strava-and-g...

[varun]

@MattS_bsb if you can file a Support ticket with your C2 rowing files, we can take a look into what is going on there, as it is likely (as you and others in the linked post noted) different than the "rounding" issue here.

[Silentvoyager]

Varun, Thank you for the detailed answer! I think Garmin does a proper rounding and not always rounding up. In other words, 4.995 rounds to 5.0 but 4.994 rounds to 4.99. Personally I think that is the way to go. That is the proper scientific approach that everyone gets taught in school. Always rounding down is arbitrary, especially considering that GPS distance error is typically much greater than the rounding error. Because of that the notion that a user might not have run the distance is simply wrong. Rounding down is even more annoying in some parts of the UI that show only one decimal digit, for example in the week stats or in the training calendar. 

[Rogier]

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.

[CreakyCrank]

I actually disagree - I think rounding down is the correct approach, rather than rounding up.  Think of it in terms of a race... if your Garmin is rounding up, and you were just about to cross the finish line, would you stop and complain that you are 'done', even though you haven't crossed the line?  Rounding down ensures that you go the FULL distance, not just shy of it.  

Another analogy is in engineering/construction - would you want something to be slightly too small, and compromise the integrity of a structure?  By rounding down, I feel that Strava is maintaining the integrity of the distance reported.

I also recognize that I may be in the minority opinion here - and these are just my opinions.  I too have finished a race (1/2 marathon) to find the distance wasn't long enough, then restarted the Garmin and (looking like a fool) ran around after the finish line to get to the proper distance.

Win Win Option? Maybe Strava could "round down" in "race mode" and then "round up" in non-race mode?  Kind of like it removes the stop time, but this would be a code change, and would also then require recalculating all activities to the current date (not an insignificant amount of computing power)... I'm not sure the cost would be justified for such a little thing, but maybe it could be done going forward... just my 2 cents.

[Silentvoyager]

Neither rounding down nor rounding up is correct. It should be a proper scientific rounding as I shown in the example above. And regarding your race example, for most consumer grade GPS devices distance accuracy is 0.5-1% at best, so let's not pretend that we don't want to give someone an extra 0.005 mile that they haven't actually run. 

[varun]

@Silentvoyager @CreakyCrank I appreciate both of you adding your perspectives, and recognize athletes will continue to disagree on the approach taken. One additional point regarding consumer GPS precision - while it is true that it tends to be in the range of a few meters (and right around 0.005 miles), there is research supporting that in many cases, GPS tends to overestimate true distance. Here are a few links discussing this:

• https://spectrum.ieee.org/why_every_gps_overestimates_distance_traveled
• https://www.tandfonline.com/doi/full/10.1080/13658816.2015.1086924
• https://www.singletracks.com/mtb-gear/do-gps-units-always-overestimate-trail-distances-no/ 

That said, we acknowledge that the approach we are taking here is as much to do with the philosophical reasoning @Silentvoyager mentioned as well about the integrity of "running through the line" as it is the science, and likewise both open and happy to hear more thoughts about this from other members of the community!

[Silentvoyager]

> 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.

[varun]

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.