SFR surface density vs. atomic hydrogen surface density. Red squares represent galaxies with v _circ < 120 km s ⁻¹. Blue triangles represent galaxies with v _circ > 120 km s ⁻¹. The circular velocities are from H i rotation curve fits (Paper I). A representative error bar is shown in the lower right corner (see Sections 3.1 and 3.2 for details). The vertical dashed line is at 9  M _☉ pc ⁻², the typical maximum density for atomic hydrogen. For comparison, the small black points represent measurements from Bigiel et al. (2008) from seven spiral galaxies sampled at 750 pc resolution.