Residual of Φ(400 pc)−Φ(0 pc) with respect to a fitted exponential, projected on the (X, Y) plane. Each panel corresponds to one of our four data samples. In each panel, dotted grey lines show galactocentric iso-radial and iso-azimuthal lines, and the black plus sign in the centre marks the solar position. The grey contour lines correspond to the spiral arms as mapped by Poggio et al. (2021), as seen in Fig. 6.