Layout of the HEALPix pixels on the sphere in a cylindrical projection and a demonstration of two possible pixel indexations—one running on isolatitude rings, the other arranged hierarchically or in a nested tree fashion. The top two panels correspond to ﹩N_{\mathrm{side}\,}=2﹩ in first ring then nested schemes; the bottom two panels are for ﹩N_{\mathrm{side}\,}=4﹩. θ refers to the colatitude and φ to the longitude.