Demonstration of the strong-selection masking in the KiDS+KiDZ catalogues for KiDZ pointing KIDZ_333p0_1p9 in the Y-band. All sources detected by SOURCE EXTRACTOR are shown coloured by the number of missing photometric bands as determined by GAAP (0: blue, 1: red). The mask is determined by the fraction of sources per unit area on sky that are missing one or more bands, as determined using a ratio of KDEs constructed on a 1′ × 1′ grid with a 1′ Gaussian kernel. The strong-selection mask removes regions with missing-source fractions greater than 20%. The effect of this mask can be seen in the background, which shows the sum image of this field’s Y-band data after masking: areas that show the strong selection effect (i.e. red dots) have a zero value in the sum image (i.e. white), and are therefore masked.