CMDeviceMotion yaw values unstable when iPhone is vertical

Question

In a iOS prototype I use a combination of CMDeviceMotion.deviceMotion.yaw and CLHeading.trueHeading to make stable compass heading that is responsive and accurate. This work

Answer 1

The problem is a bit confusing because there are at least two different ways to think about Yaw. One is from the phone's perspective, and one from the world perspective.

I'll use this image from Apple to explain further:

If the phone is flat on a table:

• Rotations along the phone's yaw (or Z axis): change the compass heading.
• Rotations along the phone's roll (or Y axis): do not change compass heading.

If the phone is flat against a wall:

• Rotations along the phone's yaw (or Z axis): do not change the compass heading.
• Rotations along the phone's roll (or Y axis): change the compass heading.

If you want the phone's compass heading while vertical

Use blkhp19's code above which sums up yaw and roll. If you import SwiftUI, you can leverage the Angle struct to make radian + degrees conversion easier:

let yaw = Angle(radians: deviceMotion.attitude.yaw)
let roll = Angle(radians: deviceMotion.attitude.roll)

var compassHeading: Angle = yaw + roll
if roll.degrees < 0 && yaw.degrees < 0 {
  compassHeading = Angle(degrees: 360 - (-1 * compassHeading.degrees))
}

If you want rotations along the phone's yaw axis while vertical

You'll need to use atan2 and inspect gravity as in this example.

let rotation = atan2(data.gravity.x,
                             data.gravity.y) - .pi

Also note that if you don't need the actual angle, and all you need is the relationship (e.g. isPhoneUpright), you can simply read gravity values for those.