mm) between electrode locations is proportional to their angular distance, which is readily expressed by the cosine of the angle between any two surface points (i.e., radians). For example, for a unit sphere (radius r = 1), the spherical distance between T7 and T8 equals π (i.e., 180° or half the circumference of a great circle), π/2 between T7 and Cz (90°), or π/8 between T7 and C5 (i.e., 90°/4).8 It is therefore of critical importance to correctly assign the measured surface potentials to their (approximate) surface location, applying either a template for standard 10–20 system locations (and its 10–10 and 10–5 extensions; cf. Jurcak et al., 2007; Oostenveld and Praamstra, 2001) or specifying locations via any coordinate system for three-dimensional space (Cartesian, polar, spherical), all of which can be easily converted to reflect unit sphere surface locations.
