FLASK-generated uniform random field, binned hexagonally (cell scale ∼ 10 arcmin) in RA/Dec and coloured according to our various artificial systematic-tracer variables. A-types vary smoothly over large angles, B-types vary over 1 deg² tiles with a 2D Gaussian form, and C-types are single-valued for each tile. Iso-contours (white lines) are lain over A-type variables for clarity, and to illustrate the subtle difference between A2 and A3.