Question about GPS distance calculation

[Wlzzz]

I'm not sure if this is the right place to ask, but this question is more directed to the developers.

So I'm interested that how the distance is currently calculated from the GPS data. I ask this because I'm seeing wrong distance calculated from data that seems to be just fine.

Here is my scenario:

I went to ski and started hike activity with connected GPS from my Charge 2.

I know that the route which I traveled is about 7km long.

At the end of the route I stopped the activity and it shows that I have traveled 4km distance. So 3km are missing.

I went home and checked the GPS data from map and it seemed just fine. No huge gaps or anything in the data.

Next I downloaded the TCX file from that exercise which contains the GPS data in xml format.

The TCX file seems to contain each GPS coordinate it takes during the exercise. Also there seems to be the distance in each trackpoint element. In the end of the file the distance is this incorrect 4km.

So next I wanted to do little test that what kind of distance I get if I use that GPS latitude and longitude data and calculate the distance myself.

So I wrote little python script which parsed each latitude and longitude element from that file. Then I used haversine formula  to calculate distance between two GPS coordinates. So my script iterates all GPS longitude and latitude points and calculates the distance between them and adds them together in one variable.

And what result did I get from this experiment? 7km.

So now I'm really curious that where this distance variable in that TCX file comes from? Is it calculated by the Fitbit App or does this come from phones GPS service?

[LanuzaFitbit]

Hello @WayneLocklear.

Thank you very much for your replies and for sharing those additional details! 

I'd like to verify that the Ionic is on the latest firmware version. To check if there is an update, and update, you can find instructions on this help article. 

Additionally, could you check if this is happening retroactively to previous activities? Perhaps that could provide details of when this started happening for you.

I look forward to your reply. 

[WayneLocklear]

Yes the iconic is on the latest firmware, first thing I checked

I've only had the iconic for a couple weeks, so the retroactive question really doesn't apply.  I can say that I took another hike today and it was accurate using the same test proceedure (another device running gaiagps)

[LanuzaFitbit]

Hello @WayneLocklear.

Thanks for sharing that you have checked on it. 

I will be sharing these details with the rest of the team. As soon as we have any news or updates on the situation we will be sure to let everyone know about it.

Feel free to reach out if you have any other questions on the matter.