Sorry for doublepost, but I want to open this problem with new informations. The strengths of GPS signal seems to be related to LTE/4G switched on. When LTE/4G is switched off, the accuracy seems to be much better. Please test it!
See this (from Apple discussion):
In France we also have this problem of GPS interference on iPhone 6S/6S+. This for now only happens with mobile operators that use specific frequencies for 4G network. If you search for "4G GPS interference" you'll see it's not a new fact ...
A temporary solution is to set the iPhone in 3G when you use your GPS, otherwise GPS signal goes on and off for any application after 15 or 30 min.
Some people reinstalled the iPhone or made an exchange of the iPhone, but this was useless.
No solution has been given to us for now, mobile operators have reported the problem to Apple but we don't have any more information