Multilevel
This app displays relative angle difference between two or more devices, making it possible to perfectly align physical objects in relation to one another.
Measuring relative levels comes in handy for aligning items that have significant distance or physical obstruction between them. Sometimes there’s a need to determine angle between separate objects, rather than see how flat they lay relative to the planet’s surface. This also comes in handy when the ground is not level, e.g. on the water or while moving.
The app uses Bluetooth to broadcast its magnetometer’s data and to observe position values of other devices that run Multi Level, then it uses that data to compare and display its own physical orientation in relation to other devices. The communication is done without having to pair the devices — there’s no need to tap on anything or deal with PIN codes, everything happens automatically.
With only one device running the app, Multilevel works as a conventional bubble level. Position measurements can be observed from desktop and laptop computers using the macOS version of Multi Level.
You won’t need to purchase this app more than once, as long as all devices use the same Apple ID.