Get reference coordinate system of FIT file

Hello everyone, 

In receive a Garmin forerunner 55 and I want to make a data visualizer from the FIT file. 

I used the python SDK to parse the FIT file. However, I cannot really understand in which coordinate reference system 
are the latitude and longitude. At first I though there were in EPSG:3857 with a factor of 10, but when I plot the final result
on a map, the track is off by ~300 kilometers. 

How could I interpret thoses values ?

Thanks a lot, 

Simon