online compass device orientation
This compass reads the device `DeviceOrientationEvent` to estimate which way the screen is pointing, and the explanation below separates what the sensor actually measures from what a real magnetic compass does.
This value alone does not say whether the device points north, east or any other direction; it only describes how much it has rotated since the sensor started running.
This proprietary WebKit property already delivers a compassed bearing in degrees, but it is still referenced to magnetic north, not true geographic north.
On iOS, this method only works when called from a user gesture, like a button tap; called outside a gesture, the browser silently rejects it.
Only if the computer and browser expose orientation sensors. Most desktops and laptops have no magnetometer, so the compass reading is not available on them.
Because iOS 13 and newer require the site to call `DeviceOrientationEvent.requestPermission()` from a user gesture, like a button tap, before releasing orientation sensor data; Android usually does not require that explicit confirmation.
Not on its own: `alpha` is relative to the device starting orientation, not to geographic north; actually pointing at north requires the absolute event or, on iOS, `webkitCompassHeading`.
The sensor measures direction relative to the Earth magnetic field, that is, magnetic north; true geographic north differs from it by magnetic declination, an angle that changes from place to place and shifts slowly year over year, so the raw sensor reading is not the same as the north on a map.
It is not recommended: beyond the uncorrected magnetic declination offset, smartphone sensors suffer interference from nearby metal and electronics, and the raw `alpha` reading is not always calibrated against north; for navigation where accuracy matters, use a dedicated magnetic compass.
Orientation is read locally; no sensor data is uploaded.