A five points stencil (2D) located close to the boundary. The central cell has two neighbours that are not masked m > 0 and two cells that are masked m < 0. One of these cells is completely masked m = −1 meaning that it does not exist in the memory, and its field value must be interpolated from the coarse grid. For the cells that are only partly masked − 1 < m < 0, the field value is determined from Eqs. (40) and (41). The cells with m > 0 are treated as normal cells even though the boundary might cross some part of the cell.