Distance measurement

[John-Walker]

I am looking to replace my aging Blaze device. 

My one issue with the Blaze is that while the GPS is used to measure how far you cycle, it uses an arbitrary calculation of step X stride length to decide how far you have gone on a run or walk exercise program. 

I don't know about you but my stride length changes day to day. In fact if I am not feeling it on my runs it can change step to step. 

Add to this that the more you swing your arms the less steps it counts (you can miss a third of your steps if you swing your arms too much).

With this in mind the blaze is useless for trying to monitor improvements. A mile today could be three quarters of a mile tomorrow and a mile and a half the day after. 

So, does the sense still use this highly inaccurate algorithm or does it use GPS to accurately measure how far you have gone in an exercise program?

Or should I end my relationship with Fitbit and start looking elsewhere?

[Gr4ndp4]

Hi, @John-Walker, welcome, inevitably newer devices have moved on from the "Blaze", you may find these articles helpful How accurate are Fitbit devices? & How does my Fitbit device calculate my daily activity? which have sections on the issue you raise.

Cheers

[t.parker]

@John-Walker distance measurement is a lot more complicated. Using GPS wouldn't work for real-time metrics. Steps x stride wouldn't work either. I believe "steps x stride" is just a very simplified scenario given to users without getting much into detail. The reality is that the stride should be recalculated in real-time based on the number of steps and data from all available sensors and corrected with GPS data. In fact, all the sensors lack accuracy in certain areas and the data of one sensor corrects the data of another sensor. I think Sense works no different but this hasn't been ever confirmed by Fitbit.

Using "steps x stride" would be an incredibly inaccurate method (even with GPS correction) so I don't believe Fitbit would use such abad distance computation method. Here's why:

White - Stride length, Blue - pace, Pink - cadence

This is my today's 5k Parkrun race. I tend to keep cadence fixed most of the time so if my pace changes, the stride length changes, too. It is easy to see that pace and stride length are quite similar and the only thing that affects it is elevation and slight changes in the cadence. Runners who keep cadence variable (most of the runners as I don't think there are many runners who use a metronome) will make stride length even more varying. So stride keeps changing and is being recalculated all the time.

Using GPS only wouldn't work either. It doesn't even work for cycling but because bike riders tend to ride longer distances and use roads it is less prone to errors. Also, when cycling, Fitbit doesn't really provide any metrics that cyclists would want to pay attention to during the ride (and how would one do it if the watch is on the wrist, right?). So GPS-only solution seems to be good enough (cyclists who care about accuracy use speed/cadence sensors rather than GPS). But it isn't for running because running needs a different level of accuracy.

Green - where it should be, Red - GPS route

As you can see above, on the short distance the GPS may be very inaccurate and as much as for the whole track the GPS will probably be ok (more or less) that will not provide correct real-time metrics.

The solution used by most of sports watches is to use all the available data (accelerometer, gyroscope, GPS, and even magnetometer to know where's the north). The data will self-correct.

We won't know what Fitbit really does but neither "steps x stride" nor "GPS" methods will provide accurate distance and real-time metrics. This is a lot more complicated and I believe Fitbit's algorithms do a lot more (whether correctly or not, we will never know).

From my experience, Sense isn't too bad computing distance although usually, it slightly underestimates it (from my last half-marathon it chopped 1km).

[John-Walker]

Thanks for this. It goes into a lot of detail. 

I know from experimentation with my Blaze that it only uses steps X stride for distance in the walking and running exercise programs. It becomes quite obvious when you do the same walk (and now run) every day and get a discrepancy of over a mile over 4 miles.

I am not a great runner having only been doing it for a couple of months so I am not after extreme accuracy. But what I would like is to know that mile 1 today is the same as mile 1 yesterday so when I get a sub 12 minute mile I can actually be proud of myself and not be wondering if I'd only done .9 of a mile. 

Do you use a Sense to track your runs?  Are you happy with it?

[t.parker]

@John-Walker I use Sense only for some runs (mostly for races). Usually, the distance is close enough. However, it's not my main watch. This probably answers your question about whether I'm happy with it (for me it's just way too basic for running).

[John-Walker]

Thank you. I think that answers my question about whether it's time to move on from Fitbit.