GPS Accuracy

[Amazza]

Accuracy on the Sense 2

So I went on a lunch time walk (blue route) however my fitbit recorded something quite different (the red route)The GPS route is well off. I didn't start the walk via my fitbit, it was automatic but I wouldn't have thought this would have had any impact on accuracy - except missing out the beginning / end etc. 

I've went from the Versa 3, which I think worked great, to the Sense 2. Even when I log runs (where I start and stop manually) the GPS is all over the place, however not as bad as the walk above. Its is after my run when I stop it, the map shows it still continues for a bit. 

I bought the Fitbit through someone else so only have 30 days to return not the 45 that fitbit offer. 

Anyone have any ideas if how I could improve the GPS accuracy? Its unusable at the moment. Is it a Sense 2 issue or just my device.  

[t.parker]

The straight line segment usually means that you lost the GPS signal somewhere. You can see that in the elevation plot, too (there will be a straight line from one point to another). The curved path near the start and finish is something that I have noticed in some of my runs. There is something I started calling "initial drift" as it turns out I see it quite often. Basically, after acquiring the GPS signal the initial coordinates are off and gradually are coming back onto the path. I haven't seen that on the original Sense (or any other device).

In general, I don't have lots of issues with GPS on Sense 2 but occasionally it drifts away (top-right picture), it almost always has the wrong starting position and drifts into the path and occasionally loses connection (straight line). In this particular case (bottom picture) I ran under the bridge so it's usual for all my devices to lose connection here. Nothing wrong with Fitbit but posted this picture to show an example of "losing GPS straight line" which your case seems to have plenty.

What can you do? If you have a clear sky over you then probably nothing.

[Amazza]

Thanks for your response. Very helpful there.

I'm trying to improve the accuracy of my distance and thus be able to calculate my average pace etc. So as long as my distance is fairly close to reality it doesn't matter really about what the map GPS looks like,. 

When looking at the total distance for the run or walk, I've read this isn't based on GPS info alone and also takes into account the steps and stride length of the user. Is this noted anywhere officially by Fitbit? Just trying to find out more information really. 

I was going to calculate my running and walking strides and update this info in the settings in the hope of improving my distance accuracy? My 5K park run was most recently 3.22 mile when my Versa 3 normally logged this as 2.91 mile.

[t.parker]

@Amazza the distance is a tricky thing. Basically, most watches (that includes Garmin, Polar etc.) use a hybrid method in order to compute running distance. Relying on the GPS only wouldn't be accurate because a little drift would make your metrics useless. Here's an example:

Orange - with GPS drift, Red - fixed (reference)

This added over 3km to my run due to really bad GPS drift (and broke my Strava PBs because Strava uses only GPS routes to estimate PBs and segments, it was tricky to get that corrected). This is a huge inaccuracy but GPS in general isn't accurate enough for slow-moving objects which humans are. Hence, watches must use a different method to estimate running metrics. Usually, it involves all the sensors (accelerometer, gyroscope, altimeter, GPS and even magnetometer in order to tell where north is). Sense 2 has a very limited choice of those sensors but the idea is that all the sensors correct each other. It isn't known how Fitbit does estimate distance. We know it uses stride length which is computed from the GPS data (and bad GPS data may trash the stride length, it happened to me in the past and I think Fitbit cannot verify the correctness of the GPS data). Shortly, the bad map is also bad data which will eventually contribute to the distance computation.