Self-similarity of the Hi column density profiles. Upper panel: all 28 Σ_HI profiles scaled in radius and normalized in amplitude. The thinnest lines correspond to the early-type spirals in our sample. The thick black line shows the clipped average profile and the dashed lines indicate the rms scatter at each radius. Lower panel: the crosses follow the thick black line in the upper panel and the dashed lines are identical to the dashed lines in the upper panel. Gray scales indicate steps of 0.5 times the rms scatter. The solid black line is the best-fit Gaussian function following the crosses above (Σ_HI/.