I have the same problem: the GPS accuracy on my iPhone 7 is really bad.
I used to track my run with Strava with my iPhone 6 and realised it was not working anymore after I get my iPhone 7.
I thought it was the app but noticed other GPS-based apps were not working (Apple Maps, Google Maps, etc.).
Here is a photo of my iPhone 6 (left) compared to my iPhone 7 (right). Same condition (Wi-Fi off), same carrier, same OS version (10.0.2). Look at the horizontal and vertical accuracy, it's as if my iPhone 7 was not receiving any GPS signal (only carriers). I really don't know what's wrong and if it is a software or hardware issue.
I will try to get an appointment at an Apple Store asap.