Top panel: Absolute value of the S_ij matrix elements in the case of a stage-IV angular mask, as predicted by PySSC in black solid line and AngPow in red dashed lines. The elements are ordered column by column of the lower half of the matrices, including the diagonal (i × i). The corresponding x-coordinates, i × j, label the redshift bins that are cross-correlated. Panels 2 to 9: Relative deviations (in percent) between the two methods for decreasing survey area. The grey area depicts the 5% discrepancy.