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#107: CF Data Model 1.7
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Reporter: markh | Owner: cf-conventions@…
Type: task | Status: new
Priority: medium | Milestone:
Component: cf-conventions | Version:
Resolution: | Keywords:
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Comment (by jonathan):
Dear Jim and Mark
`grid_mapping` and `formula_terms` look rather different in CF-netCDF, but
they do have a similar purpose. `formula_terms` tells you how to translate
the vertical coordinate variable into coordinate values which "indicate
the location of the data" (as it says in 4.3) in the vertical dimension,
in practice meaning height or depth with respect to a geophysically
defined reference surface, or pressure, which is (in some cases) a
reasonable proxy for height or depth. `grid_mapping` tells you how to
translate the horizontal coordinate variables into longitude and latitude,
which have a special status in CF for providing horizontal location; as
you know, we insist that it must be possible to locate the data in lat-lon
if it has horizontal dimensions.
After `grid_mapping` was introduced, it was expanded to allow the
ellipsoid to be defined, and as a result of recent discussion I've
proposed it should allow the geoid to be identified too. At the moment,
the ellipsoid and geoid definition cannot be applied to vertical
coordinates, but it seems very likely this will become desirable, also as
a result of recent discussions. For example, we can specify a vertical
coordinate as height above the geoid, but we cannot specify precisely what
the geoid is for the vertical coordinate, unless we allow `grid_mapping`
to do that. So, in this respect too, vertical and horizontal coordinates
will require similar treatment.
Given these two sorts of similarity, it seems logical to us that we should
describe the two CF-netCDF mechanisms as different applications of the
same logical construct. It doesn't matter that they are formally
different. The formal difference arises partly because `formula_terms`
applies to only one dimension but `grid_mapping` to two dimensions, and
partly because they were designed at different times without seeing the
larger picture. In writing down the logical model, we can take a step back
to see that larger picture, and make it simpler as a result.
Cheers
Jonathan
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Ticket URL: <http://kitt.llnl.gov/trac/ticket/107#comment:66>
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