Distance tracking way off

[magdalena_]

 This is an identical, literally identical run. Fitbit seems to think there was about a kilometer of difference between the two runs... why is this so far off?

[emili]

is this a tracker with built in GPS or connected? What was the step count for both days? I don't have an answer sadly, just looking for more data so maybe we can come up with something. 

[Heybales]

The algorithm will attempt to put your route on a known road/trail for visual aspect.

But for distance it will use the GPS data.

Depending on where in the sky the 4 required satellites were can effect accuracy.

Corners and bends easily add more.

On Garmin it was interesting to put the raw GPS data that lead to distance estimate up against the line on the road visual.

Best was it being shifted sideways only by some feet.

Worst was it bouncing back and forth to extremes of the accuracy constantly. Looked like a very drunken run.

You might want to pause prior to starting just a couple minutes in clear sky until signal accuracy is best it's going to get. Start moving prior to best fix and it can take a long while to improve.

[davidcop]

In runs today.. was trying to specially do a 5k distance so, I drove the route twice (once before and once after) and what should have been ~3.2 miles.. fitbit showed as less than 5k while running, and guessing I ended up running another half mile or more to get it to 5.2x k (and the x is because in the summary list it shows 5.21, but when you click on it it's 5.26.   On other occasions using a different app on the phone with gps and fitbit also tracking.. the distances have been somewhat different to quite a bit different (less on the fitbit) which didn't think about it too much since at the time was more using the other app.   Also, was extremely frustrated today not being able to see the charge 3 screen out in daylight trying to read the distance.

[Kate721]

This explains a lot. Yesterday I took my kids on a hike. According to MyFitnessPal it was 5.6 miles. According to Fitbit with the GPS tracking it was 8 miles. 

the interesting thing is that we usually do a 3.8 mile walk during the week (that I’m sure of the distance cause it’s marked in tenth of a mile intervals). MyFitnessPal is usually under saying it’s only 2.84 miles and Fitbit is usually accurate. 

but I also know that hike yesterday was NOT 8 miles. 5-6 miles, yes. But not 8 miles. I had my 4yo with me and there’s no way he walked 8 miles in under 3 hours. 

im thinking it’s maybe cause we were in the woods and not on any kind of known road? Or cause we were up and down hills? 

[Heybales]

Woods for sure causes accuracy issues. GPS accuracy range can be as good as about 10 ft, but on up to 150 ft easily. Start throwing around some 150 ft jumps of accuracy and distance builds up if no extra data smoothing takes place.

Vertical changes are about 3x worse accuracy than the horizontal, but not used on distance anyway for that reason.

Fitbit without GPS is using per step impact data to estimate distance, and if your stride length setting is decent that can be very accurate. Extra weight on your back compared to your weight it knows about will throw that off though.

Perhaps a factor on the hike.

How are you getting MFP to display a distance on it's own - are you selecting your phone as step source? Or you don't have your MFP-Fitbit accounts synced, but you did select Fitbit as step source?

Just curious on that since a couple ways to do it.

[Junkof]

Mine is also way off despite using Sense 2 which supposedly uses a "more accurate" watch GPS instead of phone. The faster I run, the shorter the distance recorded even for exact same distance. Anyway I "fixed" it to some extent by measuring my stride length and inputting it to the app. How to measure stride length: 
1) Go for a run on a route whose distance is easy to measure accurately via Google Maps. 
2) Look at my Fitbit to see exactly how many steps it took me to run that exact route
3) Calculate # of meters run divided by # of steps. Result is stride length

[Triletics]

@Junkof that works only (kind of) for similar runs. When you run on a variety of terrain and elevation then setting a single stride length won't work. There is a crucial difference between Fitbit and others in how they use the stride length. Fitbit assumes a fixed stride length while other solutions treat stride length as output, not input. Here's an example from two different platforms and the same run:

As you can see, other platforms MEASURE your stride length as an output metric. The white line on Stryd is a stride length. This plus the other sensors' input contributes towards distance measurement and the GPS is used to validate that. Fitbit does it differently and GPS doesn't play much role in the distance computation (if at all). You can clearly see that when you download the TCX file and calculate distances between GPS waypoints - it does not match the distance Fitbit reports.

So, the workout above was hill repeats and reported an average stride of 0.96cm (lots of running uphill and downhill, a total of over 400m elevation gain with flat warmup and cooldown). Now, see another example, this time, the tempo run (steady pace, flat paved surface):

Stryd reported 1.11m and Garmin 1.12m. 0.01m is an acceptable margin of error. Now, if I set 0.96m for Fitbit, then this particular run (1:06:57, 195 steps per minute, total of steps: 13065). Let's simplify and just multiply steps by stride length. For 0.96m, the distance would be 12542m, and for 1.115m (average stride length using both, Stryd and Garmin outputs) the distance of the same run would be 14567m. It's a 2km of difference! It means I could not use 0.96m measured during the first run to measure the distance of the second run and vice versa. If you run always the same route, the same way, similar average cadence then the Fitbit method will work. If you run many routes, alter elevation grade, intensity, surface type, etc. then the Fitbit method is no way to go. I have this problem constantly and there is no way to fix it unless Fitbit engineers implement it differently. Fitbit likes to reinvent the wheel. The problem however is that Fitbit's wheel isn't round.