Compilation of all 28 Hi column density profiles, divided in four non-equal bins for each of four global optical properties; morphological type, central disk surface brightness, absolute magnitude, and color. The profiles are scaled in radius by R_HI. The horizontal dashed lines are drawn at the sample median of 4.3 ℳ_⊙ pc^-2 as a reference. The vertical dotted lines are drawn at the radius where the average profile reaches its peak column density.