I got several devices with digital compasses: Cellphone, P4, Garmin hiking GPS, and a couple of tablets. Sometimes they are off 90 degrees for some odd reason (e.g. North pointing East) and it takes me several attempts to get them pointed north. Could be my waving them wrong antics or some interference somewhere. Probably could leave them alone once they are calibrated, but last night my iPad came up with some "Compass calibration required" message out of the blue. I think I was on Google Maps, and it took me several tries to get it aligned, but still the iPad's compass circle isn't filled in 100%, maybe only 75% which I don't know why. It points North, or close enough to it.
Mack