I find the explanation of the GPS orientation vectors (rot1 and rot2) very confusing.

Current:
For more general applications, the user may supply two vectors (x’ and a vector in the plane x’-y’) to rotate the co-ordinate axes of the shape with respect to the overall co-ordinate system (Fig. 3). 

Proposed replacement text:
For more general applications, the user may define a rotated coordinate system for the source shape by supplying two vectors defining the x'-y' plane of a new coordinate system. These may have any length, and are not required to be perpendicular to each other, provided they are not collinear. The new z'-axis direction is uniquely defined as perpendicular to this plane and oriented according to the right hand rule (Fig. 3). 

The current illustration in Fig 3 is difficult to interpret and visually ambiguous. I am attaching a proposed graphic to replace it.