# Solved – How to assess accuracy of phone GPS in measuring distances

I am a complete stranger to statistics (apart from mandatory courses in college), but lately I ran into an interesting real-world scenario.

Recently I started jogging. I take my GPS phone with me to measure time and distance. I picked a route (about 4km, but I don't know exactly) and started running. I have observed that the distance measured is not constant, although the route is the same. Sometimes, the distance measured is a little more than 4km and sometimes a little less. I am interested in the precision of my phone GPS.

But there is one small problem: when I arrive, I usually (but not always) check the phone, and if the distance is less than 4Km, I continue running to reach 4Km. So even though I have all the measurements saved in the history, I cannot use them to calculate the mean value, because sometimes I run a little bit more to reach 4km. What I do know, however, is that during my 20 runs, the minimum distance was about 3.9km and maximum 4.05km.

So what I am interested in is how precise my GPS is and how long the path is.
I expect the answer to be something along the lines "with confidence X%, the path is Y km long and the GPS has Z% distance variation". Or would I have to know the exact mean to calculate that?

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