I have done two runs, on the same course, both lasting for 30 mins, and which I used my FitBit to track - FitBit was then transferred to Strava.
I will try to attach the summaries and maps, showing the aerial view. (Though I'm limited to one photo per post, so will see if I can put the others onto replies.
Both runs were a "there and back", turning round when we heard the C25K bell.
Now, the start and end points are more or less the same, and the point we turned round at was virtually the same.
Yet, there is a difference of 0.5km between the two! The first one was basically bang on 5km, which was great news to us as we are on the last week of C25K. But today's run was 4.52km. It's a 10% discrepancy!
If anything, we ran to a further point on the shorter run. The start/end points for both are basically the same.
The GPS is not pinpoint, we ran forward and backwards on the same paths, yet the lines deviate a bit. But not by as much as that, I would have thought!
Does anyone knowledgeable in this community have any advice to give? Thanks.
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doonhamer74
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Thank you for the kind words! We were really happy to have "done" 5km, but then to find out that it might be 4.5km after all kind of felt like a kick....
I think at week 9 we’d barely run 2.5k so don’t let that kick in the …. but having said that, take the longest one, for timings Strava always makes me a little faster than Garmin which cheers me up
I've never used a Fitbit, only Strava. As I understand it Strava uses GPS to track, whereas Fitbit estimates distance on the basis of steps. On that basis, I wouldn't trust Fitbit. That said, I find that Strava is often more generous re distance than my Garmin watch!
Thank you. With Fitbit, I open an option on the watch, then wait until it says it has synced to the Fitbit app on my phone. Then my run will tie GPS to the heart rate and step info from the watch. It's the phone that provides the GPS input to Fitbit. That makes me think that it would make no difference what app I used as it will all just be from the iPhone GPS service.
Maybe. All I know is that if I record a run on Strava via my (Android) phone, and on my Garmin watch, there is almost always a discrepancy. Strava usually thinks I've run further, Garmin tends to be more generous re elevation.
Tbh it doesn't matter anyway for C25K. The point is to run 30 minutes continuously at a conversational pace, so if you're achieving that, you're already doing well, and getting to 5k is just a bonus!
My run today as follows:
Garmin GPS watch:
5.03km time 31:52.
This transfers over to Strava and becomes:
5.06km time 32:09
No idea why it changes transferring data.
Nike Run Club using Samsung phone GPS presumably
5.13km time 32:28
Although slightly different they are in the same ball park. Sometimes though, when I used my fitbit watch to record onto Strava it was as a rule of thumb about 0.3km less than NRC.
I find Strava the most accurate...no discrepancy says if like it is. Sadly sometimes for me when I come up short on a run🤣 My Garmin and it..exactly agree.
As you can see they all measure slightly differently. Today NRC registered 5.89k and my Apple Watch registered 6.06k. Just choose one and stick with it or it’ll drive you mad! 😂😂
Thank you. In my case, I'm sticking to the Fitbit/Strava combination. My problem is that for the same course, in virtually the same start/finish/turnaround points, there is such a discrepancy on the day!
My distances were so different from my daughter's, even when we were out together, that I don't rely on my Fitbit/phone at all, but plot my route on Google maps or mapometer on my laptop to see how far I've gone. But then I don't really look at pace or other stats so I don't have to do lots of calculations. I did notice that my Fitbit's estimate of distance was much closer to the Google maps figure when I walked my usual route - I'm by no means a fast runner, but I do run faster than I walk, and I think the slower pace allowed the GPS to register my position more accurately.
Thank you! I did not realise that Google Maps had the function to measure a multi-point track! With this, I've been able to measure out 2.5km... and I think that the "turn round" point is further on than where we turned.
Oh well, at least I know now to treat the Strava measurement as "probably optimistic". Thought I do wonder if the 4.5km report was more accurate..
I run with my son and daughter. All having the same fitbit. My son is nearly always a good 2k more than me and my daughter is always 1k behind me and we run exactly together.My daughter gets so upset over it and she shows poor times.
It really isn't worth getting upset and if you want to get accurate 5ks you can join your local parkrun.
We started a couple of weeks ago and they're a far better way to measure times and distance if it matters to you.
Me, I just enjoy the run and don't worry too much about the stats! 😁🏃♂️🏃♀️
I do intend to do my first Parkrun soon! After I complete my final C25K. 😀
I don't want to get stressed about it, but I don't like to not understand things. Now, I know that I have to give a good pinch of salt to what is reported.
I have ran in the past with a friend of mine. I have a Garmin Forerunner 35 watch and she has a Fitbit. We have ran at parkrun, same course, same distance and measured using a proper professional wheel. We have clicked go at the same time and crossed the finish line at the same time pressing stop at exactly the same time. Mine is alway accurate to within a few meters. But my friend is always out over 5K by as much as 500 meters just like you said. Garmin measures via GPS tracking whereas her watch calculates distance via steps. GPS tracking on phones will always be a little bit different to GPS tracking on a running watch, but are closer.
Thank you. the 500m discrepancy over 5km does indeed sound familiar!
I don't know what type of Fitbit your friend has. Mine is the Inspire HR. This watch contains no GPS itself.
The default option is that it estimates distance via steps, but if, before I tell it that I have started a run, I open the app on my phone, then wait till it confirms that it has synced, Fitbit then uses GPS for the distance. That's what then gets sent to Strava.
If I don't do this - or if I mess up the syncing, Fitbit still records that I have done a run, but there is no map to view - and it does not tell Strava about my run.
My Garmin vivoactive3 always under-records my local parkrun (lots of tree cover to muck up the GPS signal, lots of 90 degree bends) by about 10% (500m in 5km).
I wouldn't be concerned. Even with an excellent signal, GPS tracks can wander by as much as 3m in any direction.
There's a lot of complicated maths involved and it depends on a lot of factors which can affect how the signals from the satellites behave.
Often a smartphone has a more capable GPS receiver than a watch, though.
Reminds me of a programme I saw where they were trying to measure the coastline of Britain and two ways differed by 100s of percent. The problem was any distance measured is the sum of a number of straight lines, even where it winds all over the place. It’s probably the same when we run: we run in straight lines even going around bends. In essence, the shorter the straight lines, the more accurate the measurement is. But the computation takes longer because there’s more lines to add up.
Anyway, there’s a number of factors at play. Both my wife and I walked hand in hand - ahh! - and we both recorded it on Strava. She has an iPhone, I have a Samsung (android) and the difference was 7.5km (me) and 9.2km (hers) which is a ridiculous discrepancy.
When I zoomed in on the maps, hers zigzagged down a road whereas mine more or less showed a straight path.
I now use a Garmin Fr45 and don’t record on Strava but it syncs afterwards. I’m guessing the raw data is exchanged and each processes it in a different way.
Today’s run tallied okay except for the elevation gain. That’s a puzzle.
Garmin: 8.35km, 47:26, 5:41/km, EG 106m
Strava: 8.34km, 47:27, 5:41/km, EG 127m
I’m still getting used to the watch and I see there’s different GPS modes I can set. No idea which is the best or whether one is more reliable than the others.
The two runs I posted both had wiggling, though looking at it, I could not see enough difference between them to explain the 10% discrepancy.
I would have thought the companies would "smooth it out" a bit. I mean, it knows that the activity is a human being running, so big leaps and jumps are an indication of GPS inaccuracy, rather than me actually doing big zig zags. Oh well.
And your elevation discrepancy is indeed worrying. It also shows how much flatter my chosen course is than yours! 😜😜
You and your wife had a 22% discrepancy! That's really worrying and makes me inclined not to trust what is reported...
There are a number of factors that affect the accuracy of the GPS location, route and distance covered.
Firstly, the accuracy of the the location at any one time is determined by how many satellite signals can be received, any less than seven will not give a very reliable location. Depending on where you run will determine how many satellites can be detected. The GPS receiver measures the difference between the time it receives each signal to determine the position. The accuracy of the devices internal clock will also affect the accuracy, a cheap device may well have a less accurate clock and so give a less accurate position. A good demonstration of the issues this causes can be seen if you use a tracking application while on a treadmill, the result will usually show quite a bit of movement even though you stayed in the same position. The same app used on different devices is likely to give different results.
Secondly, the tracking applications are not monitoring the location continuously. They check the location at intervals, the more often, the more accurate, but also the more power is used. There is a trade-off between accuracy and power use. This means that running two or more tracking apps on the same device over the same route will possibly give different results.
The route will also affect the accuracy. If you ran in a straight line the sample rate will not make much difference as the distance between the two samples will be correct. However, if you ran in circles, the samples taken assume a straight line between the two points and so the more often the sample is taken, the closer the distance will be to the actual distance.
Devices specifically designed for GPS tracking (i.e. Sat Nav's, sports/smart tracker watches with built in GPS etc.) will generally be more accurate than multi-purpose devices such as Smartphones. The technical specifications of the device will probably quote details of its GPS receiver.
I originally had a FitBit Blaze which uses the phone GPS and although I ran exactly the same route each time, the distance recorded varied. I now have a FitBit Ionic which has built in GPS and is extremely consistent on the same route, I can even see on the map where i run into the road and around cars to avoid pedestrians which shows the sample rate is quite high.
Thank you very much for that detailed explanation.
That contradicts one other suggestion on this thread, i.e. that the phone's GPS would be more accurate than would be the case of a tracker watch fitted with GPS.
Thinking about it, it should be that a decent sized watch would be designed with tracking in mind - and therefore more accurate than a multi-purpose phone.
I have a Fitbit Inspire HR. It's really interesting to read your experiences between the Blaze and the Ionic.
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Can I call myself a Trail Runner now? 
Some firsts 😱😄😉
Do I believe my Fitbit or my Nike run app for calories burned ?
Coach Cory and 10k rows
I have no idea of how far I am running 🤔
