# 4.2. Definition of the grid

There are 3 possibilities. 2 are always possible, one is available only if the PGD routine is integrated into an atmospheric model initialization facility.

1. The grid is chosen via namelists options (see below)
2. The grid is defined as a part of the grid of an already existing surface file, indicated via namelists (see below)
3. The grid is defined as being identical to the one of an atmospheric model, which is given as fortran argument in the coupling of the PGD surface facilities (routine PGD_SURF_ATM) into an atmospheric model initialization procedures. In this case, all namelists that are usually used to define the surface grid are ignored. Note that, in addition to the grid, the orography can also be given from the atmospheric file.

Note that all the namelists presented in this section are ignored if the grid is imposed, in the fortran code, from an atmospheric model. This is the case when one already have defined the atmospheric grid and one want to be sure that the surface has the same grid. For example, this is what happens in the MESONH program PREP_IDEAL_CASE (when no physiographic surface file is used).

If you are in this case, ignore all the namelists presented in this section, and only the namelists for cover and the following ones, have to be used.