Example of the HOG method morphological correlation between the 3D dust density reconstruction and the line emission observations for the region 30° < l < 40° and |b| < 5°. Each panel corresponds to the correlation between distance channels, from the 3D dust density, and velocity channels, from the H I and CO line emission, shown in the left and right panels, respectively. The correlation metric is the direction-sensitive projected Rayleigh statistic (V_d, Eq. (2)). Values of V_d ≈ 0 correspond to a random orientation between the gradients of the two tracers, thus indicating low morphological correlation. Values of V_d > 2.87 correspond to mostly parallel gradients, thus indicating a significant morphological correlation. Values of V_d < 2.87 correspond to mostly antiparallel gradients. The white markers indicate the v_LOS with the highest V_d for each distance channel. The green dashed lines correspond to the distance and v_LOS with the highest V_d for the whole velocity and distance ranges. The three cyan lines represent the kinematic distances for the Galactic longitude at the center and the region’s edges, according to the Reid et al. (2019) Galactic rotation model.