Correlation and pixel-to-pixel comparison between HFI and SPIRE observations of the Hi-GAL L59 field at 545 GHz. Top left: HFI and SPIRE maps of the L59 field. Top right: SPIRE–HFI pixel-to-pixel correlation. The linear fit from the unweighted “X on Y” regression is plotted in red, with the 1-to-1 line in black for reference. The offset removed from the SPIRE map during the map-making has been obtained from the fit and added back. Bottom right: horizontal residuals from the fit, as a function of brightness. Bottom left: map of the horizontal residuals, with a colour range that is ten times finer (centred on 0), to enhance the details.