Longitude-velocity (lυ) diagrams of the filament orientation quantified by the projected Rayleigh statistic (V, right) in the two portions of the Galactic plane covered in the range |b| ≤ 10° by both HI4PI and GALFA-H I. TheGALFA-H I observations were analyzed at 1.29 and 0.184 km s⁻¹ resolution using a 5′ FHWM derivative kernel. The curves correspond to the same spiral arm features introduced in Fig. 1.