Kinect 2 Inconsistent FloorClipPlane reading

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C#

Kinect

SDK

I have successfully implemented the floor clip plane to measure the distance of left foot to the floor, which is fairly accurate. The problem I have is that as I move away from the camera (i.e. left foot Z axis is increased), the foot distance to the floor changes (increases).
Note: The floor itself is not tilted nor the Kinect stand.
I tested it with Kinect 1 and had the same result. The subject's head height (Y axis) also changes value as I move away or get closer to the camera. It does not matter of the camera is tilted or line of sight. the D value in the FloorClipPlane equation shows a constant number during the test.

A = bodyFrame.FloorClipPlane.X;
B = bodyFrame.FloorClipPlane.Y;
C = bodyFrame.FloorClipPlane.Z;
D = bodyFrame.FloorClipPlane.W;


distanceLeftFoot = A * leftFootPosX + B * leftFootPosY + C * leftFootPosZ + D;


double divisor = Math.Sqrt(leftFootPosX * leftFootPosX + leftFootPosY * leftFootPosY + leftFootPosZ * leftFootPosZ); 


float temp=Convert.ToSingle(divisor);


distanceLeftFoot = (A * leftFootPosX + B * leftFootPosY + C * leftFootPosZ + D)/ temp;

Just to let you know, I have coordinate mapping between depth and colour. Not sure if that has anything to do with the issue. I am not sure if it is because of lens distortion of Kinect 2 due to its wide angle view.