Batschelet, Edward. Circular statistics in Biology (1981) Academic
Press Inc., New York, NY. This method is used for averaging angles
that can encompass a full 360 degrees. For motivation, think of
what the average of 0 and 359 degrees or 0 and 180 degrees would
be. The 'averages' from this method are in the range [-180..180],
so e.g. a single value of 183 would result in a statistical 'average' of -177.
Another reference for circular statistics mentioned
on the net as being easier to find:
Fisher, N. I. Statistical Analysis of Circular Data (1993)
Cambridge University Press, New York.
Kabsch, (1976) Acta Cryst. A32, 922-923 and
(1978) Acta Cryst. A34, 827-828.
\