Noise level of rescaled KP₅₁₂, KP_SPM and MDI 5-min magnetograms, and of HMI 315-s magnetograms (taken from Yeo et al. 2013) as a function of position on the solar disc. The circular area covered by each colour scale plot corresponds to the part of the field-of-view occupied by the solar disc and the surrounding box the boundary of the field-of-view. The contours correspond to the first, second and third quartiles, and the crosses to the minimum and maximum points. The contour and point labels are in units of Gauss. We omit the second quartile in the KP₅₁₂ and KP_SPM plots to avoid cluttering. The KP₅₁₂, KP_SPM and HMI noise surfaces were resampled to MDI image size (1024 × 1024 pixels) to allow a direct comparison.