Parallel deviation is computed using the following steps:
Ignoring midside nodes, unit vectors are constructed in 3-D space along each element edge, adjusted for consistent direction, as demonstrated in Figure 47: Parallel Deviation Unit Vectors.
For each pair of opposite edges, the dot product of the unit vectors is computed, then the angle (in degrees) whose cosine is that dot product. The parallel deviation is the larger of these 2 angles. (In the illustration above, the dot product of the 2 horizontal unit vectors is 1, and acos (1) = 0°. The dot product of the 2 vertical vectors is 0.342, and acos (0.342) = 70°. Therefore, this element's parallel deviation is 70°.)
The best possible deviation, for a flat rectangle, is 0°. Figure 48: Parallel Deviations for Quadrilaterals shows quadrilaterals having deviations of 0°, 70°, 100°, 150°, and 170°.
