Longitude-velocity (lv) diagrams corresponding to the mean intensity and the results of the Hessian analysis of the velocity-channel maps in the region covered by the THOR observations. Each pixel in the diagrams represents the quantities calculated in 2° × 2° tile in a 1.5 km s⁻¹-velocity channel as follows. Top: mean HI intensity (I). Middle top: mean resulting vector (r). Middle bottom: projected Rayleigh statistic (V). Bottom: mean orientation angle () of the HI filamentary structures, which is only well-defined if r > 0. The overlaid curves correspond to selected spiral arms from the model presented in Reid et al. (2016). The vertical line indicates thecentral l of the 2° × 2° tile presented inFig. 1 and whose circular statistics are shown in Fig. 3.