How Accurately Do Running Apps Track Your Distance?

I trust my running app, most of the time. If I set out for a three-miler, and come home with only 2.8 on the screen, I’ll jog around the block to finish the job. But even a smartphone can be dumb sometimes: like when MapMyRun grabbed a data point on the other side of the river my friend was running along.

Source: How Accurately Do Running Apps Track Your Distance?

Running, cycling, swimming… All the same issue.  Aside from marathon training, most advocate “double the distance”.  Which covers the margin of error…

Don’t forget about running tangents!