Illustration of the individual coordinate system.

The longitudinal (ξ_i) and lateral (η_i) axes are determined by the momentary motion of the i-th individual. (ξ_ij, η_ij) denote the coordinates, ψ_ij the polar angle of the j-th individual in the ξ_i − η_i plane. This figure is a modified version of Fig 3 of [10].