This generally happens because of bad GPS data.

On Strava you have the options to Correct Elevation, Correct Distance or Crop the activity. These all work if you had a good GPS signal.

The one-sentence answer: Rounding errors! Now, the geek squad explanation: Most bike computers, and I assume watches, record distances in meters, and display distances in the user's chosen unit of measure, metric or customary. Normally the display of kilometers or miles presents only two digits to the right of the decimal point, so a rounding takes place internally making the result less precise, but more manageable for the average user when they view it on the screen.

Most devices and activity tracking websites take that file made by the GPS unit and round up. However there's one website that takes the file provided and rounds down. You guessed it: Strava is the one rounding down.

I see the above scenario happening to me almost every time I ride. I can hit pause & save on the Garmin unit at 37.01 miles. The Strava activity page will show exactly 37.00.

The above are not GPS errors, but there are some instances where you and your riding companion can record differing distances even though you both pressed start and then save at the exact same times. Two devices can utilize differing methods of recording distance (wheel speed sensor vs. GPS only.) Some environmental factors provide a weaker satellite signal than others. A cheaper, earlier device may not write a datapoint to the file upon pressing save, relying only upon the last recorded point, even though it may have occurred a few meters back... thus robbing the user of that distance. There are probably about 12 or 15 oddities like these that may not be apparent every time you ride or run.

The rounding method I mentioned above is the primary bugaboo you must compensate for in doing a competitive event. You must go a bit beyond what your devise displays to assure you are credited by Strava with the required distance.

Sources:
https://support.strava.com/hc/en-us/articles/216919487-How-Distance-is-Calculated

https://communityhub.strava.com/t5/strava-features-chat/why-strava-distance-is-always-0-01-mile-shorter-than-garmin/m-p/535

https://support.garmin.com/en-GB/?faq=P3DdzRfgik3fky125aHsFA&productID=552982&tab=topics&topicTag=region_gps

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Discrepancies occur when the device used isn't a Strava Trusted Device. This happens a lot when a manufacturer releases a brand new model that Strava doesn't yet have it in their database. An example I was personally involved with was the first edition of Garmin Enduro APAC version. I reached out to Strava, submitted several FIT files and once Strava's databased was updated, data synced across matched especially for models that have built-in barometer where the recorded Elevation Gain numbers are also synced without correction. 

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Thank you to those who participated in our first Wisdom Wednesday series!

Congrats to our randomly selected winner, @JBW-Florida for their correct answer! We will be reaching out to you regarding your free, one-month subscription.

While JBW - Florida’s answer is correct, another factor that could have resulted in varying data is the distance stream.

Not all activity files will provide distance information. If the activity file does not provide this information Strava will calculate its own distance based on the available GPS data. Some files do include distance information, but if the device isn’t recognized by our system, Strava will also calculate its own distance based on the available GPS data.

If your activity file includes distance information and is recorded by a recognized device Strava will use the distance that is in the file. Keep in mind that the distances stored in the GPS file are stored in meters, so if you’re still seeing a very slight difference it is likely due to a rounding discrepancy when converting from meters to miles or kilometers.

If you’d like to know if Strava is calculating distance or if we’re using the distance that’s in the file:

• Go to your activity page on the website (this feature is currently not accessible in our mobile apps)

• From the activity page click on (◦◦◦)

• Go to your activity page on the website (this feature is currently not accessible in our mobile apps)


• From the activity page click on (◦◦◦)


• If you see the option to “Correct Distance”, it means the activity is currently showing the distance that was included in the uploaded file.

• If the “Correct Distance” option is missing, it means that Strava is calculating the distance based on the available GPS data.


• From the pop-up, if you click “Correct Distance”, the activity distance will then be calculated by Strava using the available GPS data


• When the status changes from “Calculating” to “Updated,” refresh the page.

Change your mind? That’s OK! Simply follow the directions above again and select ‘Revert Distance’ and the original stats will be reinstated.

Next Wednesday, September 27th, we will pose another question for your chance to win a free one-month subscription! Thank you to everyone who participated. Happy Trails!

Thank you very much.  It's been a while since I won something in cycling.  I already got my voucher for the one-month membership.  Thank you again.  

JBW