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    <p>The question would not have arisen if CMDeviceMotion had an accessor for the userAcceleration in coordinates of the reference frame. So, I used a category to add the required method:</p> <p>In CMDeviceMotion+TransformToReferenceFrame.h:</p> <pre><code>#import &lt;CoreMotion/CoreMotion.h&gt; @interface CMDeviceMotion (TransformToReferenceFrame) -(CMAcceleration)userAccelerationInReferenceFrame; @end </code></pre> <p>and in CMDeviceMotion+TransformToReferenceFrame.m:</p> <pre><code>#import "CMDeviceMotion+TransformToReferenceFrame.h" @implementation CMDeviceMotion (TransformToReferenceFrame) -(CMAcceleration)userAccelerationInReferenceFrame { CMAcceleration acc = [self userAcceleration]; CMRotationMatrix rot = [self attitude].rotationMatrix; CMAcceleration accRef; accRef.x = acc.x*rot.m11 + acc.y*rot.m12 + acc.z*rot.m13; accRef.y = acc.x*rot.m21 + acc.y*rot.m22 + acc.z*rot.m23; accRef.z = acc.x*rot.m31 + acc.y*rot.m32 + acc.z*rot.m33; return accRef; } @end </code></pre> <p>and in Swift 3</p> <pre><code>extension CMDeviceMotion { var userAccelerationInReferenceFrame: CMAcceleration { let acc = self.userAcceleration let rot = self.attitude.rotationMatrix var accRef = CMAcceleration() accRef.x = acc.x*rot.m11 + acc.y*rot.m12 + acc.z*rot.m13; accRef.y = acc.x*rot.m21 + acc.y*rot.m22 + acc.z*rot.m23; accRef.z = acc.x*rot.m31 + acc.y*rot.m32 + acc.z*rot.m33; return accRef; } } </code></pre> <p>Now, code that previously used [deviceMotion userAcceleration] can use [deviceMotion userAccelerationInReferenceFrame] instead.</p>
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    1. COThis is interesting, nice work. I assume you are using it in conjunction with the compass (CLHeading). What are you doing with the numbers you get from the acceleration?
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    2. COIn theory, initialising with CMAttitudeReferenceFrameXTrueNorthZVertical means that the compass heading is already considered. In general, there are many interesting uses for the world acceleration rather than device acceleration.
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    3. CO@Zaq Can i use this concept to rotational rate data(rotate.x,rotate.y,rotate.z) and for the reading of yaw of the iphone? Thanks in advance!
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