Distance is the most basic training statistic - therefore, being confident in your distance data is important. There are a few ways to gather distance data while recording a GPS-based activity
- How Strava measures and displays distance
- Methods of Calculating Distance
- Reverting Distance
- Recording distance on a GPS device
How Strava measures and displays distance
When a GPS file is uploaded, Strava takes the distance data recorded in the file and parses it into a data stream to calculate total distance, average speed, and max speed. Depending on which method for recording distance is used (see explanation below), that data will be reflected in the distance stream, and thus on Strava. Under normal conditions, differences should be minimal when comparing distance or speed metrics on Strava versus the GPS device, but any small inconsistencies are likely due to number crunching on both ends - Strava processes and analyzes the data in the file independently, whereas most GPS devices tabulate these values on the device itself.
On Strava, distance contributes to your overall distance totals, whether it is in your Training Calendar, the bar graph on the Profile page, or in your overall and yearly stats in the Profile sidebar. Additionally, distance readings contribute to your average speed, as average speed is calculated from distance over your total moving time. Distance does not, however, contribute to your segments or segment times. Segment times are based on when you cross the start and endpoints of a segment. Therefore, distance is mostly a personal metric and statistic, except when Strava runs a distance-based challenge, like the monthly distance challenges, and in that case, distance would be competitive.
Methods of Calculating Distance
There are two main ways to calculate 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 that is 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 method of calculation does not capture variations in the route between GPS points and may vary further when battery-saving features are enabled.
- GPS-based, Strava post-upload approach: After GPS data is recorded, and uploaded to Strava, the data is parsed into streams of data and analyzed. At this time, a calculation can be run on the GPS coordinates to get distance. This is how Strava determines distance for any uploaded file that does not include a distance stream. You can elect to use this method if you suspect there is a problem with your device's recorded distance (see reverting distance.)
- Pros: Post-upload GPS-based distance can eliminate problems like stuck points (see above) and create smoother, more accurate distance data than the device equivalent.
- Cons: A flat surface is assumed, and vertical speed from topography is not accounted for. Similar to the above, straight lines connect the GPS points.
- Speed/Cadence Sensor Garmin GSC-10 approach: Ground Speed distance is measured by counting the revolutions of the wheel, and then multiplying by the wheel circumference.
- Pros: A wheel sensor will capture vertical speed and the additional percentage of distance accumulated with changes in elevation. For mountain bikers who gain and lose a lot of elevation gain rapidly, this could become a slightly more significant factor.
- Cons: Common problems with relying on a wheel sensor include: wheel size is not documented accurately, the device is moved to another bike with different wheel size and not adjusted, the Auto wheel size is calculated wrong either because of GPS inaccuracies or because the magnet did not count every wheel revolution. See this article for how to ensure you get the most accurate distance data and the correct wheel setting.
Reverting your Distance
If you suspect there is a problem with the distance recorded by your device you have the option to override your device's distance with the Strava post-upload approach. Click on the "Distance?" text under the distance statistic in the overview. A dialogue will pop up with a button to "Revert Distance." This can improve the quality of uploaded data by eliminating outlier GPS data like inaccurate GPS points and data that is clearly inconsistent with the file. If you change your mind, click the text again to revert back to the original distance.
Hierarchy of Garmin Device inputs when multiple Distance data sources exist
What happens if you have a power tap or a GSC-10 Speed/Cadence sensor or both? When the Edge has multiple sources for the same information it uses a predetermined selection process to go with what it considers will be the most accurate source.
- If you have a Power Tap hub connected to your Gamin, it will take speed readings from the Power Tap hub above all other inputs.
- If you have a GSC-10 Speed/Cadence sensor, the Garmin will take readings from this output over the GPS-calculated distance.
- If you have neither, the Garmin will calculate distance based on GPS.
The key is that the data from either of these sources are seamlessly incorporated into the recorded file, under the distance stream. In some cases, the speed in MPH is documented in the file also, as an extension. Regardless, each Garmin-produced file has a distance stream of data measured in accumulated meters that serves to measure total distance and speed (both max and average).