Projected Rayleigh statistic, V, the statistical test of spatial correlation between HI and ¹³CO velocity-channel maps in the HOG method, as defined in Eq. (3), after flipping the velocity channels maps in the following directions. Top left panel: original THOR HI and GRS ¹³CO tiles. Top right panel: original THOR HI tile and GRS ¹³CO flipped in the vertical (galactic latitude) direction Bottom left panel: original THOR HI tile and GRS ¹³CO flipped in the horizontal (galactic longitude) direction. Bottom right panel: original THOR HI tile and GRS ¹³CO flipped in both the vertical and horizontal directions. The contours correspond to the 3ς_V, 4ς_V, and 5ς_V values in the indicated velocity range. The large differences between the top left and the other panels indicates that the effect of chance correlation in the results of the HOG method is small.