Grids and Remappers

In EAMxx, the AbstractGrid is an interface used to access information regarding the horizontal and vertical discretization. The most important information that the grid stores is:

the number of local/global DOFs: these are the degrees of freedom of the horizontal grid only. Here, local/global refers to the MPI partitioning.
the DOFs global IDs (GIDs): a list of GIDs of the DOFs on the current MPI rank, stored as a Field
the local IDs (LIDs) to index list: this list maps the LID of a DOF (that is, the position of the DOF in the GID list) to a "native" indexing system for that DOF. For instance, a PointGrid (a class derived from AbstractGrid) is a simple collection of points, so the "native" indexing system coincides with the LIDs. However, for a SEGrid (a derived class, for spectral element grids), the "native" indexing is a triplet (ielem,igp,jgp), specifying the element index, and the two indices of the Gauss point within the element.
geometry data: stored as a std::map<std::string,Field>, this represent any data that is intrinsically linked to the grid (either along the horizontal or vertical direction), such as lat/lon coordinates, vertical coordinates, area associated with the DOF.

Grids can also be used to retrieve the layout of a 2d/3d scalar/vector field, which allows certain downstream classes to perform certain operations without assuming anything on the horizontal grid.

In general, grid objects are passed around the different parts of EAMxx as const objects (read-only). The internal data can only be modified during construction, which usually is handled by a GridsManager object.