Column densities for the 4π sr as seen by a cell sitting on the edge of the sphere (12.5,25,25) [ pc ]). The abscissa corresponds to the angle φ (from 0 to 2π), and the cosθ (from −1 to 1) is given in the ordinate. The reference maps (on the left) have been calculated using the gather approach. The panels at the center present the maps calculated using the tree-based method, while on the right we present the error maps calculated according to Eq. (12). From top to bottom: using 12, 40, 84, 144, and 220 directions.