It is very heavily recommended to opt in your users in this service (and mandatory to ask the user's permission for the location detection anyway, the most cost

It is very heavily recommended to opt in your users in this service (and mandatory to ask the user's permission for the location detection anyway, the most costly by far privacy & battery wise). I am not sure how legal it would be to collect this kind of data without asking for the user's approval first (not that it would stop all devs of course). Glancing at the doc, I might be mistaken, but it looks like on iOS, just like on Android, you don't need to ask a permission in order to access to the accelerometer.