Distance is the most basic training statistic, so 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
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. Still, any minor inconsistencies are likely due to number crunching on both ends, since Strava processes and analyzes the data in the file independently. In contrast, most GPS devices tabulate these values on the device itself.
On Strava, distance contributes to your overall distance totals, whether in your Training Calendar, the bar graph on the Profile page, or your overall and yearly stats in the Profile sidebar. Additionally, distance readings contribute to your average speed, as average speed is calculated from a 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, in which case the distance would be competitive.
Methods of Calculating Distance
There are two main ways to calculate distance in most sports: Ground Speed Distance and GPS-Calculated Distance. Ground speed measures your speed along the surface you are traveling (by counting the revolutions of a wheel), and GPS-calculated distance connects the dots between your GPS points and triangulates 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 accumulated distance in real time while the device is recording, based on GPS data. Pros: a refined calculation gathers distance data built into the file's 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 after 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 account for variations in the route between GPS points and may further deviate when battery-saving features are enabled.
GPS-based, Strava post-upload approach: After GPS data is recorded and uploaded to Strava, it is parsed into data streams and analyzed. At this time, a calculation can be run on the GPS coordinates to get the distance. This is how Strava determines the length of any uploaded file that does not include a distance stream. You can use this method if you suspect a problem with your device's recorded distance (see "reverting distance" below). Pros: post-upload GPS-based distance can eliminate issues like stuck points and produce smoother, more accurate distance data than the device's 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 wheel revolutions and then multiplying by the wheel circumference. Pros: a wheel sensor will capture vertical speed and the additional percentage of distance accumulated with elevation changes. This could become a slightly more significant factor for mountain bikers who gain and lose significant elevation rapidly. Cons: common problems with relying on a wheel sensor include inaccurate wheel size documentation, the device being moved to another bike with a different wheel size without adjustment, or the auto-detected wheel size being calculated incorrectly due to GPS inaccuracies or because the magnet did not count every wheel revolution.
Reverting Your Distance
If you suspect a problem with the distance recorded by your device, you can override it using the Strava post-upload approach. From the Strava website, click on the three-dot action menu in the left sidebar and select the option "Correct Distance." This can improve the quality of uploaded data by eliminating outlier GPS data like inaccurate GPS points and data that is inconsistent with the file. If you change your mind, click the button 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 PowerTap 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 will consider to be the most accurate source.
If you have a PowerTap hub connected to your Garmin, it will take speed readings from the PowerTap hub above all other inputs.
If you have a GSC-10 Speed/Cadence sensor, the Garmin will use this sensor's output for distance rather than GPS-calculated distance.
If you have neither, Garmin will calculate the distance based on GPS.
The key is that the data from either source is 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 tracks total distance and both max and average speed.