Surface density distribution for Gaia DR2 and the new external catalogues (see Sect. 4) obtained using a HEALPix (Hierarchical Equal Area isoLatitude Pixelization, Górski et al. 2005) tessellation with resolution N_side = 2⁸ for dense surveys and N_side = 2⁶ for sparse catalogues. In grey we indicate areas that are not covered by the survey. The surface density distribution of external catalogues that were also matched with Gaia DR1 can be found in Paper I.