Forecast errors on cosmological parameters for different mask geometry and a 15 000 deg² survey, for GCph+WL+XC, when marginalising (a) or not (b). Top: relative error as a percentage for each cosmological parameter. The filled coloured bars represent the constraints obtained with the psky derivation, using a circular mask (blue), a mask divided into two circles (orange), or a stage-IV-like mask (green). The hatched empty bars are the ones obtained in the fsky approximation. Bottom: relative difference between the fsky and the psky standard deviation for each mask geometry.