Illustration of the method used to simulate a mock weak lensing survey. 2D maps of the correlated 3 × 2 pt fields are generated at finely sampled points in redshift. These 2D maps are used to approximate the full 3D cosmological information of the 3 × 2 pt signal. From the overdensity fields, we Poisson sample a galaxy population that traces the underlying n(z) distribution. We then assign the correlated weak lensing observables to each galaxy from the shear field values at the galaxy’s angular position on the sky at a given redshift. We show an early approximation of the Euclid DR1 footprint at the bottom of the figure which we have used to create our Euclid-like simulations. The ‘observed’ region is shown in yellow. Note: the actual Euclid DR1 footprint will be significantly different to that shown here.