Bad GPS Data: What/Why/How

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?

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.

Col_de_Pantol___Strava_Ride-1.jpg

 

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.

Heifer___Strava_Ride-1.jpg

 

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.

Morgendliche_Radfahrt___Strava_Ride-1.jpg

 

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.

As for specific devices; the device manufacturer will always have the most up-to-date and relevant information, but you can see our Knowledge base articles for Android and iPhone troubleshooting. 

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Comments

21 comments
  • Questions or feedback about this article?  I'm interested in making sure these are as helpful and informative as possible - so if you have any thoughts please feel free to comment.

    Keep in mind, however, that this not the best place to request support - for that please create a support ticket.

  • Hey Gordon... Yes this article is very usefull. Unfortunatelly took me too long to come to the support for an answer to my issues on the App... many times my rides were not being tracked, probably because these issues... I went to my earlier rides and the gps stopped working right in front of a tall building near my home :/. I usually leave my Android inside my bad, so I think this is the issue. The android is ussualy in the middle of work cloths. I'll try and leave it as high as possible inside the bag.

    Thanks for the article ;)

  • Today on my run, I had the app close for some reason in the middle of it, then I looked to check my distance, and noticed it had cut out. It did ask me if I want to continue, but now there's a straight line from the last GPS fix to when I restarted it (similar to "bad GPS data" problems). This is more of a device issue than the app, but there should be a way to correct the missing data, even if that means being disqualified from any segments that happen to be a part of the corrected portion.

  • Thanks for your thoughts, Matt.  That's not specifically in the plans at this point, but it would be nice to be able to represent that portion of your activity.  For now, you have the option of adding a manual activity to account for that distance, or if you were running/riding with someone, you could ask that person if they are willing to share their GPX file with you.

  • iPhone is notoriously bad at recording GPS. Not sure why.  I also find that most errant segments on Strava are by people with iPhones...  

    I use Garmin for road cycling and it is very good with no errant points.  Garmin is notoriously bad while mountain biking when you have tight technical turns it is not registering all of the turns also challenged by trees and cliffs...  So it can really vary by device... in my experience.  I use my Android phone too sometimes for running and mountain biking as it has more ways to lock signal (satellite+towers+wifi).  People tend to have these data problems with particular devices that do not have good firmware with their GPS or are possibly outdated.  This was a very useful post as it helps to educate and opens up the wonderful sophistication of Strava that we appreciate very much.

     

  • Chris - I use a Garmin and use the setting where it records data every second; This setting may help with data loss on technical tracks. You're also right about the firmware updates being an important component.

  • One of your competitors allows you to modify the route post ride, ie manually drag the "straight line bad data" portion to fit the road. This would be a great feature to have for when GPS is bad or, as it quite common, you forgot to restart the GPS after stopping and only realise 3 miles down the road!!!

  • Is there a way to modify time or distance?  I ran a half-marathon today using my Garmin, but the distance recorded was 20.7 km and so it does not qualify in my records.  The problem was that a short portion of the run was in a tunnel that went under a river.  The Garmin connected the dots between loosing and picking up the GPS signal, but there were short switchbacks to descend and ascend to the tunnel and these were not recorded by the Garmin.  Is there any way to amend this?

  • In some cities it might also help if the map data were leveraged alongside the GPS data.  For example, sometimes running in NYC the GPS looks like  I ran in the middle of a block.  If you make the assumption that people aren't running on the roofs of buildings (generally safe) there could be a "lock to street grid" mode which could correct some of the issues.  Even in forest examples you have here some "lock to nearby road" function during post-processing of the data might help with the data correction.

  • I had a problem of loss of distance today. We did a 19k run with the audio set to let us know each half kilometre, just as we reached our end point the alert said it was 19ks, but when I saved the data, it came up as only 18.1 ks. The time was correct. When I looked at the segment list the last kilometre just wasn't there. It was very strange and annoying. If it was just a bad data thing you have thought the regular half k alerts would have been affected?

  • Tracy; it sounds like most likely what happened is that your phone recorded some bad GPS data that added distance to your running total - but this bad data was then recognized and filtered out upon upload to our servers, causing that distance to be removed.

    Todd; there isn't any specific plan to implement a feature like this - but rest assured that we're keeping it in mind and always working on improvements.

    Sean and James; there isn't any way to modify the data that's recorded.  Sorry about that!

    Chris; you're definitely right... The more frequently your GPS device records - the better!

     

    Finally - I've left this page open for comments in order to get feedback about the article itself and how it can be more helpful; if you have specific questions about your own segments, etc. - please create a support ticket.

    Thanks!

  • I use an i-phone 4s which is generally pretty good. Today we did shuttle runs of a mountain bike downhill track that is next to the road. We did 15 runs and I have 1 of the runs with a 50s time vs all the other runs that are over 2 mins. Is there a way to remove the one faulty segment run without cropping the ride into 2 parts and cutting the bad segment run out. 

  • I'm using the iPhone app and haven't had a problem until my last 2 activities.  It is pinging GPS locations way off my path and crediting me with more than twice my actual mileage on my most recent activity.  It's very disappointing to have what appears to be a sudden change in accuracy.  Gonna have to go back to the Garmin, I guess.

  • Like, Phil, suddenly my tracking has gone crazy.  Today and yesterday, on two routes I've run dozens of times with great accuracy, suddenly four and a half miles has become 26 at an average pace of 00:01:29/mile!  Are the Russians messing with the Strava team while you're over watching the Olympics?

  • Strava GPS trace in China apparently does not account for China GPS offset problem, a well documented issue with GPS in China. Does Strava plan to address this in a future release?

  • No problems over the past year, but suddlenly tracking issues over the past week.  My runs, too, are indicating I'm running twice as far as I actually ran and at a speed that would put me on a better than world record pace!  I use the iPhone 4.   

  • Strava have you the way you calculate distance.  I and other have issues with iPhone lately.

  • I rebooted my iPhone 4s and the GPS is working well again, so far.  I encourage others to try this as well.  If it goes bad again, I'll repost.

  • thanks for the article -- I have a related issue; a friend and I regularly run / bike together and his Strava app records a longer distance than my Strava app.   For example this weekend, my mileage was 9.6M and his was 10.2 for a 10M run, we ran side-by-side and the course was likely closer to 10.2M than 9.6M.    We both have iphones.....any ideas why there is always a difference in mileage?    Most of me doesn't care, but would love more accurate data

  • For people having problems with iPhone GPS data, I honestly think it is the iPhone and not the app.  On our group mtn bike rides, the iPhone users in our group regularly record data that is different from those with dedicated Garmin GPS units, which I trust as more accurate because they rely on USGPS and GLONASS satellite networks simultaneously and also have error correction such as WAAS, which improves accuracy to within 3 meters.  The iPhones typically record longer distances, higher cumulative elevation gain, etc.

  • Hello all - 

    My intent for leaving this page open for comments was to gather feedback about the article itself, and how I might be able to make it more useful and informative.  The best way for you to get rapid and personal support is still to create a support ticket - so in light of that I'm going to close this thread for comments.  If you have any questions, please feel free to create a support ticket - and thanks for understanding.

    -Mat

    Strava Support

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