Maximum detection of the position of the phone inside the building?

Question

iPhone

Hello. Since working with GPS inside a building is difficult, a solution is being sought to help determine the position of the phone (gsm, wi-fi, etc.) as accurately as possible inside the building.
Colleagues here suggest about seamless roaming using wifi.
Who can faced a similar task?

Answer 1

GSM is not possible.
Wifi is easy. I came across, but my solution with a high degree of probability will not fit into your budget, and I did not work with cheaper implementations.