Every phone using the app can contribute to your map data. If you combine magnetic data plus wifi data plus user behavior data, I expect you'd be able to continue to provide good 3D data to users.
I'd bet the hard part is getting building owners to update things regularly. Who's going to remember to change the map when the grocery store moves the cereal from aisle 2 to aisle 7?
You raise another point I haven't thought of: 3D. People are different heights... surely that changes the fields too. Interesting problems to be overcome.
I'd bet the hard part is getting building owners to update things regularly. Who's going to remember to change the map when the grocery store moves the cereal from aisle 2 to aisle 7?