Determine your Fitbit stride length using a GPS watch

After collecting some data using my Garmin Forerunner and my Fitbit I wanted to find out if I could determine my Fitbit stride length using R:

Neat! My y-intercept is within a few meters of 0, but that’s using the distance measured with the Fitbit:

I’ve compared Fitbit to GPS distance measurements while hiking; the Fitbit number usually seemed more trustworthy.