The essentials of TMAP grids

                           VARIABLES --> GRIDS --> AXES
    The intention of the TMAP grid structures is
           o to provide a symmetrical treatment of all axes;
           o to preserve grid structure relationships from one variable to
             another (for example, models using staggered grids may have
             velocity staggered from temperature yet these variables may
             share the same time stepping structure and possibly the same
             vertical layering);
           o to separate the notion of a "variable" from the specifics of the
             grid coordinates on which it is defined - ultimately allowing
             software to make intelligent decisions regarding the regridding
             of data.

    Definitions:
    AXIS - a structure consisting of a title, units and values to be used as
           coordinates.
           o Axes may me REGULAR, IRREGULAR or DISORDERED
           o Axes have titles (for labelling outputs)
           o Axes have units (for labelling and for interconvertibility)
           o Each axis has a unique name

    GRID - a structure consisting of 4 axes, an XY plane rotation angle and
           an inner/outer product association for each axis
           o grids point to axes by name (missing axes are designated
             "NORMAL")
           o several grids can share the same (named) axis definition -
             emphasizing common structure to the grids
           o can easily represent simple rectangular grids
           o can easily represent simple tuples (2,3 and 4 dimensional)
           o can easily represent complex (but often occuring) combinations
             of tuples and rectangular grids. For example an XY scatter of
             current meters will frequently be deployed at the same depths
             and synchronized on the same time axes.  This structure can be
             represented fully.
           o XY plane rotation permits ship tracks to be recognizable as
             lines (again emphasizing underlying structure) - very useful for
             analysis. (Rotations could be generalized to include other
             planes besides XY - e.g. rotation in the XT plane to look at
             results along characteristic curves - perhaps a pointer to a
             named rotation matrix structure instead of a single angle ???)
           o Each grid has a unique name

    VARIABLE - a structure consisting of a grid, data values defined on that
               grid, a variable name, a title string and units
           o variables point to grids by name
           o multiple variables may share the same grid emphasizing structure
           o regridding is easily represented since variables (as "objects")
             are separated from grids.