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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
Replying to [comment:71 biard]:
> I think you've put your finger directly on the disagreement I have with
your approach. As I look at it, grid_mapping does not do the thing you
describe in point 1 [''i.e. georeferencing''], and formula_terms is not
involved with the thing you describe in point 2 [''i.e. defining a
geophysical surface''].
1. Latitude and longitude are implicitly georeferenced - not precisely,
without stating the ellipsoid, but with sufficient definition for many
purposes, especially in the spherical GCM world. Projection coordinates on
a Cartesian plane, however, are not at all georeferenced without the
projection information. The `grid_mapping` provides that information.
2. Vertical coordinates in CF, identified by `standard_names`, are
referred to a geophysically defined surface e.g.
`height_above_reference_ellipsoid`. For some purposes, it may be necessary
to specify precisely what that surface is. We cannot currently do this in
CF for vertical coordinates, but I think it is highly likely that we will
want to do it, since the issue has already been raised. We could do it
using a `grid_mapping`, and then a vertical coordinate would require both
`formula_terms` and `grid_mapping` for precise georeferencing.
In summary, both `grid_mapping` and `formula_terms` have function (1), for
different kinds of coordinate. `grid_mapping` has function (2), which is
currently allowed only for horizontal coordinates. This mismatch between
purposes and CF-netCDF constructs arises from the history of the
convention. If we were starting from scratch, I think we would have a
netCDF construct like `grid_mapping`, but extended, for both horizontal
and vertical coordinates, and that would simpler. Essentially, that is
what we are proposing for the logical data model.
Cheers
Jonathan
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Ticket URL: <http://kitt.llnl.gov/trac/ticket/107#comment:74>
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