Three notions of depth for directional data, angular simplicial depth (ASD), angular Tukey's depth (ATD) and arc distance depth (ADD), are developed and studied. The empirical versions of these depths give rise to center-outward rankings of angular data which may be regarded as extensions of the usual center-outward ranking on the line. Three medians derived from these depths are examined and compared. Applications in nonparametric classification and in implementing the bootstrap to construct confidence regions for directional parameters are briefly discussed.
Regina Y. Liu. Kesar Singh. "Ordering Directional Data: Concepts of Data Depth on Circles and Spheres." Ann. Statist. 20 (3) 1468 - 1484, September, 1992. https://doi.org/10.1214/aos/1176348779