Question: I recorded an activity on my device and the numbers are different after posting to Strava - Why? We’ve all been there where we have finished up an amazing ride or run recorded on our device and when it is posted to Strava, the numbers are different. What is up?
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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.
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.