Cartoon illustrating the two different binning alternatives introduced in this Appendix. Both alternatives involve defining bins along each spiral arm (‘boxes’ delimited by regularly spaced cuts perpendicular the spiral spine). In the first alternative, the corresponding interarm region is defined as an elliptical annulus covering all pixels with the range of galactocentric distances present in the spiral bin, and excluding the spiral mask. In the second alternative, the interarm value is obtained as the average surface density within two adjacent ‘boxes’ adjacent to the spiral bin and immediately outside the spiral mask.