In order to measure the distortion of the quadrilateral face of a solid element, a unique reference plane is defined by two auxiliary plane vectors. The plane vectors are calculated as:

PL1 = (V3 + V2) - (V1 + V4)

PL2 = (V3 + V4) - (V1 + V2)

where, V1, V2, V3, and V4 are the vectors that connect the four corner nodes with the centroid of the quadrilateral. These two plane vectors and the centroid are then used to construct the reference plane. These vectors of a non-warping quadrilateral face are shown below: