In a sport where fractions of a second could mean the difference between winning a race or not and training / racing distances are significant metrics, some insight regarding the use of GPS tracking watches / devices / websites / apps can be helpful to better understand the pros / cons that each offer.

To track progress, many of us turn to our devices / apps for data on the course we run / walk / race. It's important to note the differences in these tools along with their limitations.

Every device / brand tracks via GPS slightly differently!

The number of satellites used, weather (cloud cover especially), dense tree cover and the runner's route taken (USATF certification goes by the SRP (shortest route possible) if you didn't take the tangents in a curve, ran when rainy/cloudy, ran a very technical course with many tight turns...all could have an impact on the accuracy of your data collected! 

This is why, especially during longer distances...there will be variation. Ask someone who has run a marathon about the disparity of watch distances. It can be entertaining to listen to the watches / devices pinging pretty close to each other at Mile 1...but as the miles tick by, so grows the gap between one person's mile alert tone and others around you. 

Here's a great video to explain a bit more: 

According to STRAVA: ​

Methods of Calculating Distance 

There are two main ways to calculate the distance for most sports - Ground Speed Distance and GPS-calculated Distance. Ground speed will measure your speed along the surface you are traveling (counting the revolutions of a wheel), and
GPS-calculated distance will "connect the dots" between your GPS points and triangulate the distance between the coordinates. Each method of gathering data can and may introduce some inaccuracy.

GPS-based device approach: The Strava mobile apps and many GPS devices will calculate your distance accumulated in "real-time" while the device is recording based on the GPS data.

Pros: Refined calculation to gather distance data built into the file in the distance stream, measured in meters.

Cons: The complicated nature of this "real-time" calculation can lead to stuck points, where no additional distance is recorded from the previous point, which can cause some Strava calculations like Estimated Best Efforts for Run to fail. Since this is a GPS-calculated distance, a flat surface is assumed, and vertical speed from topography is not accounted for. Also, some accumulated distance may be lost as straight lines connect each GPS coordinate instead of an arc. This calculation method does not capture variations in the route between GPS points and may vary further when battery-saving features are enabled.

What about USATF Certified Courses?

These are the gold bar. Courses measured

using USATF guidelines are measured to an

exacting standard used by the world's most

demanding races. From the NYC Marathon to your local 5K, having a course USATF certified ensures the proper distance, just keep in mind...USATF Certified Courses use the "shortest route possible" and are measured first with a calibration course (defined distance measured by steel tape) and then ridden a minimum of twice using a USATF approved and 

calibrated counting wheel. If you're interested in more info on the USATF Course Certification procedures, click here!

*Not all courses are USATF certified, mostly due to expense, though race directors / timers who have been trained to do so can use the same standards / tools to ensure a correct distance.

Why is my Activity's Distance Different than my Friend's?

In most cases, this will be because one or both of your GPS devices recorded location (or other) data that does not accurately represent your activity. Things like GPS drift, GPS signal loss, or a 'jumpy' GPS track can cause your activity to report more or less distance than you actually traveled. - STRAVA 


Click on each of these industry leaders to read more about GPS accuracy: