Radial distance evolution of the opacity based on the inverted 3D maps, in the four NPS directions observed by Willingale et al. (2003) and Miller et al. (2008) (see text). The opacities are here transformed to N(H) through the classical relation N(H) = 5e21 ×E(B − V). Also shown are the absorbing columns Willingale et al. (2003) and Miller et al. (2008) deduced from the spectral modeling (dot-dashed lines), as well as the LOS integrated HI columns from the LAB survey (dashed lines).