Change of coordinate, (), from the physical spatial coordinates to the grid spatial coordinates. The grid coordinates vary in the range 0 ≤ Y ≤ M and 0 ≤ Ζ ≤ Ν, where M and Ν are two integers that do not need to be defined at this stage but will correspond to the numbers of horizontal and vertical grid levels in the finite-volume discretisation of the primitive equations. The metric is illustrated by the spacing variation between two isolines. The interval between two isolines in grid coordinates is of size 1 to mark the cells of the finite-volume method employed in the following.