Schematic drawing illustrating the concept of turbulent cells and layers used in our estimation of σ_turb,i, the uncertainty in arising from turbulence in the magneto-ionic ISM, for a particular bin i (see Eq. (A.5) and attendant discussion). The Galactic volume subtended by bin i (delimited by the four black dotted lines originating from the Sun) is divided into transverse layers of thickness l (delimited by black solid lines). These layers contain an increasing number of turbulent cells of size l (represented by opaque blue spheres, while transparent blue spheres represent cells falling outside the bin). The distance L_tr from the Sun marks the transition between the so-called nearby layers, which can be completely filled with a single cell, and the distant layers, which enclose several cells.