iOS 11 and GPS accuracy on iPhone (6+)
Try updating to iOS 12. This should hopefully provide a fix for it. Also when you try to get your location try to wait a bit as accuracy improves over time. Make sure that you've set the date, time, and time zone correctly on the device in Settings > General > Date &amp; Time. If possible, use Set Automatically.


  GPS accuracy depends on the number of visible GPS satellites. Locating
  all visible satellites can take several minutes, with accuracy
  gradually increasing over time. To improve GPS accuracy:
  
  
  Make sure that you've set the date, time, and time zone correctly on the device in Settings > General > Date &amp; Time. If possible, use Set
  Automatically.
  Keep a clear view in several directions. Walls, vehicle roofs, tall buildings, mountains, and other obstructions can block line of sight
  to GPS satellites. When this happens, your device uses Wi-Fi or
  cellular networks to determine your position until the GPS satellites
  are visible again.
  


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