Schematic (not drawn to scale) showing parts of a flat “field” (129 × 129 pixels). The covariance matrix is computed at each of the large squares, or “patches” (33 × 33 pixels), from the “background” map, and then averaged over them. There are 49 overlapping patches (7 × 7) in each field. These are laid out as shown in the figure. The full likelihood is only evaluated at the interior of the central patch (in red with the PCCS2+2E position at its centre, in yellow). The RELTH statistic is then estimated using the pixels of the red region, leaving a border of four pixels. The field/patch Y and X directions, at the centre of the field, match the Galactic coordinate lines of constant latitude and longitude, respectively. Each individual pixel (not drawn) is .