Indoor geolocation (Geolocation inside the building)?

Question

Mobile development

Interested in geopositioning options inside the building.
Conditions:

• Works on most devices;
  
• Installed from the store.
  

As I understand it, there are some difficulties with the Wi-Fi API on some OSes.
Then, how much is the organization of this process possible if there is a network of distributed Wi-Fi points?
What other options?

Answer 1

habrahabr.ru/post/143027
there are links in the comments

Answer 2

1. Triangulation (three minimum - for a plane (2D), four minimum - for a storey building 3D) by signal strength from Wi-Fi points. (connection to them is optional)
2. Collection of empirical data: accelerometer + position sensor in space (+ compass).