Velocity offset with respect to the closest spiral arm versus Galactic longitude, integrated across the whole lv-map. The colour of each pixel shows the value of the integrated flux in a bin of Δl ~ 0.5° and ΔV = 0.5 km s⁻¹. Red, blue, and cyan lines mark the ΔV values that encompass the 25%, 75%, and 95% quartile values (respectively) of the flux distribution in a given longitudinal bin. The yellow horizontal line marks the position of a velocity offset ΔV = 10 km s⁻¹, which indicates our spiral-arm velocity threshold.