S1ChordAngle represents the angle subtended by a chord (i.e., the straight line segment connecting two points on the sphere). Its representation makes it very efficient for computing and comparing distances, but unlike S1Angle it is only capable of representing angles between 0 and Pi radians. Generally, S1ChordAngle should only be used in loops where many angles need to be calculated and compared. Otherwise it is simpler to use S1Angle.