Borehole image and dipmeter derived orientation data are drawn from distributions that repeat themselves around a sphere or circle. A measure of central tendency, as an average, of such distributions cannot be derived using normal arithmetic. With such oriented data, vector means are calculated from given populations using the vector components of each data point.
SWAN, A. R. H. and SANDILANDS, M. 1995. Introduction to Geological Data Analysis. Blackwell, Oxford, p466.