Mask for the eight fields where more than 1% of the pixels were excluded (NEP = 17%, POL = 83%, SPC = 26%, POLNOR = 30%, SPIDER = 24%, UMA = 56%, UMAEAST = 66%, DRACO = 29%). For each field the left image is the total Hi integrated emission and the right image is the Planck 857GHz map. The regions in greyscale were excluded from the correlation analysis. Note the relationship of the masks to the regions of high residual in Figs. 5 to 11.