Using geomagnetic interference data for indoor positioning has already been in place for some time, but glad to see it getting more traction in adoption.

The original researchers have started a PaaS solution providing an API that you can hook into your apps today, allowing you to get (lat, lng) coordinates inside mapped structures. One needs to build path-finding on top of that though.

http://www.indooratlas.com/

Another Android app won an award from Google several years ago for allowing you to easily share directions with other people. It looks a lot like this one. You would take photos of waypoints and add them onto a map. I think it was called Breadcrumbs. Unfortunately it is no longer developed.

The easiest use case I can see is as a way for people to record where things are in a new office building, e.g., the route to the bathroom or supply closet. Then you could share the routes, though the bathroom example might not be one people want to share. Alternatively the office manager could create and share indoor routes.

This would also work well for an airport.

I went to install it on my Samsung Galaxy S5 Neo but Google Play said it couldn't be installed in my country (Germany).

The most interesting feature to me is the ability to add annotations along the way. I'd use that to describe wayfinding points, such as "the elevator" or "the giant ice cream cone."

This is strange. There should be no restrictions for Germany. We'll check that.

It should be working now. Can you confirm?

Crowd-sourcing the data is going to cost less, but this will also produce lower quality results. I think that's why all the pros use Mappedin:

https://mappedin.com/