⎡ ⎤ ⎢ ⎥ ⎢ ⎥ ⎣ ⎢ ⎥ ⎦

The parameter xxε has a relation to the direction Z of 4iP . Thus, change the coordinate Z of 4iP to obtain Equations 17 and 18:

(17)

(18)

From this, three error parameters (Sxyεxxεyy) are obtained.

3.6 Measuring along the lines in the XZ plane

The Y-axis is kept in a fixed location while spindle B2 moves along the Z-axis, and table B4 moves along the X-axis. Measure the relative displacement between the point 2iP and 4iP .