[CF-metadata] grid mappings

[Jonathan Gregory]

Dear All

My views are evidently a minority opinion so I'll vote for John and Russ's
scheme too. Anyway I feel happier about the issues now as a result of the
discussion - thanks everyone for your patience and time.

I would like to propose that the attributes defining the grid mapping, which
can only be used on a grid mapping variable, should be listed in a separate
appendix from the other attributes, for clarity. Conversely, the attributes
listed in appendix A cannot be used on a grid mapping variable.

I'd also support Brian's modification of the proposal, to use lower case for
all the attribute and mapping names, as elsewhere in the CF standard.

The grid mappings which have been requested so far are these:

  Lambert conformal
  transverse Mercator
  polar stereographic
  Albers equal area
  rotated pole

  conformal cubic
  stretched grid (as in the Arpege model)

I would suggest we start with the first five. The first four are in proj, which
we can use a source for their parameter definitions, and the rotated pole we
already have in CF. I have references for the last two, which we can add soon.
Two other possibilities are the GME grid and the CSU hexagonal grid.

Best wishes

Jonathan