Example of a source for which the HOG method indicates a high morphological correlation and the Spearman correlation coefficient indicates a low correlation. The source is AG332.7016-0.5870 and in the comparison of the continuum and of the moment-0 of H₂CO 3_2,1−2_2,0 in the mask_com. V_N=0.64, while ρ_s=0.07. The two panels in the upper row show the continuum and the moment-0 map of H₂CO 3_2,1−2_2,0 with superimposed the directions of the gradients in the two maps (red: moment-0, white: continuum, respectively). The panel in the lower row shows the scatter plot of the pixel-to-pixel rank variables in the two maps inside the same mask (in black in the images of the two maps) where both the Spearman correlation and the Projected Rayleigh Statistic, V_N, have been computed.