Having trouble with inaccurate GPS data plaguing your activities? See the information below to learn more about what bad GPS data is, why it happens, and how to prevent it.
In this article:
- What is bad GPS data
- Why does bad GPS data happen?
- Examples of bad GPS data
- How to fix bad GPS data
- How to prevent bad GPS data
What is bad GPS data?
We use the term 'Bad GPS data' to describe any circumstance where your GPS device records location (or other) data that does not accurately represent your activity. This can mean your device simply lost a connection to GPS satellites and did not record any data; that it recorded GPS points that deviate from your true path; or any number of other unfortunate experiences.
Bad GPS data can result in your activities on Strava having missing or extra distance recorded; segments not recorded on your ride or run - or recorded inaccurately; inaccurate elevation data; inaccurate achievements such as Estimated Best Efforts; and more.
Why does bad GPS data happen?
There are many factors that contribute to the accuracy of your GPS data - and it's important to keep in mind that no data is perfect; in fact there is some degree of error inherent in any GPS recording. The most accurate fitness-oriented GPS devices, at their best performance, are accurate to around three meters. Ignoring (for now) environmental factors, different devices do simply have different qualities of GPS hardware and software - meaning that even if your device is working at its peak performance, there will always be a margin of error in the accuracy of its recording.
GPS works by connecting your device to a number of overhead satellites and very precisely measuring the amount of time it takes for a signal to travel between your device and the satellites. Because the speed at which the signal travels is known, the amount of time the signal is in transit allows your device to calculate the distance to each satellite. Since the position of each satellite is known, as well as the distance between those satellites and your GPS device, your position can be triangulated. For more information see this Knowledge Base article.
Because we're dealing with extremely fast signals requiring precise detection, any slight inaccuracy in the signal's reception, or disturbance to the signal itself, can translate to a significant dislocation of your reported position. Consequently, environmental factors such as dense trees, steep hillsides, tall buildings, or even heavy cloud cover can impact or even interrupt the travel of the GPS signal between your device and the satellites.
Examples of bad GPS data
Keeping all this information in mind, lets take a look at some demonstrative examples of bad GPS data - in order to see how these theoretical factors can manifest themselves on your activities.
Below, you'll see a classic example of 'GPS Drift' on one of the Bay Area's classic climbs - one just as notorious for its propensity for GPS error as it is for its difficulty. Panoramic Highway climbs from the Pacific Ocean most of the way up Mt. Tamalpais; starting out on exposed grades and entering an area of dense, tall trees about a third of the way up the climb. As the activity below demonstrates, the GPS track follows the road with great accuracy until it enters the tree cover - at which point it begins to deviate from the road; although you can see that the route generally follows the shape of the road, it does so with much less precision. Bad GPS data like this could cause your activity to report less distance than you actually travelled, or to not match segments that you would have otherwise matched.
Another classic Bay Area climb, Old Raildroad Grade follows the contours of Mt. Tamalpais while ducking in and out of tree cover. As you can see in the activity below, these environmental factors can often cause your GPS signal to be lost and some time later re-acquired. In cases like this, the pre- and post-signal-loss points will be treated just as any other two subsequent points (although more time has elapsed between them) and connect them with a straight line. Cases of signal loss like this can cause your activity to report less distance than you actually travelled, or to not match segments that you would have otherwise matched.
For a typical example of bad GPS data caused by running or riding in an urban environment surrounded by tall buildings, see the activity below. Because tall buildings can often cause a GPS signal to 'bounce' on its way between your GPS device and the satellites, this adds extra distance - and therefore extra time - to your device's calculation of your position. This will often result in a 'jumpy' gps track like you see below. Bad GPS data like this can often cause your activity to report more distance than you actually travelled, since each 'zig' and 'zag' of your GPS track has to be accounted for with a straight line connecting them. It can, in turn, cause you to receive inaccurate achievements on your activity, or miss segments that you would have otherwise matched.
How to fix bad GPS data
While Strava does our best to optimize our data analysis by ignoring the most obviously inaccurate data, in the end we can only make the most out of the data provided to us from the GPS device. Put another way, when a device records bad GPS data, the only option that Strava has to improve it is to 'ignore' portions of that bad data; there isn't currently a way to 'correct' it. We are, of course, always working on improving our system - and as new GPS devices and hardware are released, the standard of GPS quality continues to rise.
In order to provide the best possible experience in the face of these limitations, Strava provides tools to manage the way that GPS data is represented by activities. For example, if a segment didn't match your activity, but you think it should have - you can request that the Strava Support team review and attempt to force that segment to match your ride or run. For more information on that see this Knowledge Base article. You can also, in some cases, 'correct' the elevation data calculated for your activity - for more information see this Knowledge Base article. Finally, you always have the option to Crop your activity to simply cut out the most affected parts of your activity - especially if the bad GPS data occurred at the beginning or end of your ride or run. For more information on that see this Knowledge Base article.
How to prevent bad GPS data
Thankfully, there is more that can be done to prevent bad GPS data from being recorded than there is to repair it. As previously mentioned, different devices do simply have different qualities of GPS hardware, so although there are some tips and tricks that are device-specific (more on that later), there are also some key practices that can help improve your GPS data quality regardless of what device you're using.
Sometimes it's as simple as a matter of where your GPS device is positioned; making sure your device is mounted/carried as high up and unobstructed as possible, instead of buried deep in a pocket. You can also do your best to avoid environments that are always going to be problematic for a GPS signal such as canyons - whether natural or man made (think a city street lined by tall buildings); dense trees, etc.
In many cases it will help to give your device a minute or two to fully acquire a signal before beginning your ride or run; and doing so outside, rather than inside while you get ready, will be beneficial.
Finally, you've heard it before - and as silly as it sounds... Sometimes simply turning your device off and on again, or disabling and enabling GPS, will do the trick.