Distributions of estimated statistical errors σ in SPIRE surface brightness zero points for our fields. The values indicated are the formal errors of linear fits between Herschel and the interpolated IRIS and Planck data. The estimated error in the 250 μm zero point is larger than 1.3 MJy sr^-1 for three fields. Frame b) shows the same distribution as a function of relative uncertainty, σ divided by the median surface brightness of the map.