On a recent early morning run, I forgot to take off my Fitbit Blaze and swap to my normal running watch, a Fitbit Surge (yeah – I kinda like Fitbit’s gear). As a result, I was stuck with a tracker that lacks a dedicated GPS.
The Fitbit Blaze relies on “Connected GPS”. According to the marketing material this will “map your routes and see run stats like pace and duration on display”.
So, how did it perform?
On arrival for my early morning run – a 5:30AM wake-up is my excuse for forgetting my usual running watch – I prepared the Blaze for my run. This required changing the settings on my iPhone to give the Fitbit app access to location tracking.
Once that was done, I swiped and tapped on the Blaze’s display to initiate my run. I was running a familiar route, Melbourne’s famous Tan Track, so I knew the distance I was planning to run. The plan was to run 2km between the Anderson St and Swan St bridges and two laps of the Tan.
All up, it would be somewhere between 9.5km and 10km.
On the run
By the time the watch alerted me that I’d completed my first kilometre, I knew Fitbit’s “Connected GPS” was going to let me down. Having run this route many times, I have a pretty good idea of the distances. The Blaze signalled the completion of the first kilometre about 100m short of the usual mark.
This differential of about 10% was constant for the duration of the run.
By the time I’d completed my 2km warm-up and the two laps of the 3.8km circuit, the Blaze chalked up 11km rather than the sub-10km distance I actually completed.
Validating the data
With only two real samples I could rely on, previous run data from my Surge with its integrated GPS and the Blaze and its connected GPS, I needed to see which was actually more correct.
For that, I went to MapMyRun. With this online tool (you’ll need a user account to use it) I could plot my route on a map and check the distance.
It’s important to understand that sport watches are quite variable when it comes to the accuracy of measurement using GPS. There are some good explanations for this with this explanation from Fellrnr comprehensive and comprehensible.
MapMyRun plotted the distance at 9.64km. The Fitbit Blaze plotted it as 11km.
That’s a significant difference.
Explaining the difference
I think the explanation for the difference is quite straightforward.
Although the Blaze used my iPhone’s GPS to plot where I’d run, it used the number of steps I’d taken and an estimation of my stride length to approximate the distance I covered.
I took 10,460 steps to cover the 9.64km I travelled. Some quick arithmetic suggests a stride length of about 92cm. It seems Fitbit thought my running stride length was closer to 105cm.
I’ve adjusted that setting in my Fitbit profile so, if I’m caught out again I ought to get a more accurate measure of distance.
My expectation was Connected GPS would use the iPhone’s Location Services to determine the distance I travelled rather than just drawing a map of where I’d run.
Clearly, this was a flawed assumption.