Before revision: 400px|The position angle of the point t at the point s is the angle at which the green and the dashed great circles intersect at s. The unit directions , and the rotation axis are marked by arrows.Detailed evaluation of the optimum direction is possible if the sea surface is approximated by a sphere surface.
After revision: Relation to geocentric coordinate system400px|The position angle of the point t at the point s is the angle at which the green and the dashed great circles intersect at s. The unit directions , and the rotation axis are marked by arrows.Detailed evaluation of the optimum direction is possible if the sea surface is approximated by a sphere surface.