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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 Jon
Thanks for thinking about it.
Although `formula_terms` was introduced to turn vertical dimensionless
coordinats into dimensional ones, there are cases (e.g. `hybrid height`)
where some of the input has physical dimensions, and perhaps others will
be added too. Hence its title is out of date and Appendix D should be
renamed.
The purpose of `formula_terms` is stated in 4.3.2 as to "provide a mapping
between the dimensionless coordinate values and dimensional values that
can positively and uniquely indicate the location of the data". But
dealing with "dimensionless" coordinates is not the main purpose of it, so
this statement is not quite right and not clear enough, I would say; it
could for instance be modified to read "provide a mapping between the
values of the one-dimensional variables containing vertical coordinate
values (which may be dimensionless) and dimensional values that can
uniquely indicate the vertical location of the data" to describe the
current situation.
The purpose of `grid_mapping` is described in 5.6 as "to describe the
mapping between the given [horizontal] coordinate variables and the true
latitude and longitude coordinates". Would you not agree there is a
similarity of purpose? In both cases, the 1D coordinate values are not
georeferenced, and the intention is to make a link to other values which
are georeferenced (at least approximately). I think putting together two
constructs which have a similar logical purpose, although they are
achieved with different netCDF mechanisms, is a simplification in logical
terms. What do you or others see as the logical distinction between these
purposes, apart from one being vertical and the other horizontal?
The reference ellipsoid has two distinct kinds of purpose. (1) You need it
to convert between projection coordinates and lat-lon coordinates. (2) You
need it (as you have explained) to assign the height, latitude and
longitude of any point which is not actually on the ellipsoid, since this
is done by dropping a perpendicular to the surface. We could put purpose
(1) together with `formula_terms` and the projection part of
`grid_mapping`, and identify purpose (2) as a different logical construct,
which will probably also be needed for vertical coordinate variables (that
hasn't been proposed yet). But that separation would be inconvenient
because it's the same information needed for both purposes, and the
purposes are not distinguished in CRS descriptions.
I don't think that merging `formula_terms` and `grid_mapping` into one
logical construct gives any problem for interoperability with other
standards. Relevant elements of this construct, including ellipsoid and
map projection definitions, map onto CRS descriptions quite
straightforwardly, as Etienne Tourigny showed. I agree that
interoperability is important. What specific problems can other people see
with this approach?
Cheers
Jonathan
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Ticket URL: <https://cf-pcmdi.llnl.gov/trac/ticket/107#comment:84>
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