#140: Clarifying the role of attributes on boundary variables.
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Reporter: davidhassell | Owner: cf-conventions@…
Type: enhancement | Status: new
Priority: medium | Milestone:
Component: cf-conventions | Version:
Resolution: | Keywords: boundary variable, attribute
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Comment (by jonathan):
Dear David and Karl
Thanks for doing this, David.
I agree with Karl that it would seem more obvious and usual to have the
half-levels as bounds in the examples.
The two examples are of course very similar. I can imagine a reader
glancing at them and thinking they ''are'' the same, and being
consequently puzzled, or poring over them to spot the differences, as I
have just been doing. :-) The titles of the examples are not easy to
understand, I would say, and therefore don't help very much. I suggest
solving these difficulties by having only one example, labelled something
like "Specifying `formula_terms` when a parametric coordinate variable has
bounds", in which you give the `bounds` attributes of the formula terms
variables and the `coordinates` attribute of the data variable a CDL
comment such as `// This attribute is included for the optional second
method`. You could perhaps also comment on the `formula_terms` of the
bounds `// This attribute is mandatory`.
I'm sorry to say that I might disagree with Karl's final comment. I hope
this isn't going to break our consensus! Actually I hadn't thought of this
consequence before, which relates to our different views about the paths
to formula terms, and indeed the original motive for this ticket. David
and I feel that bounds variables are closely related to their parent
variables, and the natural route to a bounds variable is via its parent;
therefore they can share attributes, including `units` and
`standard_name`. Thus Karl's requirement would contradict the first
paragraph if the formula terms have bounds, as you say, Karl. What do you
think, David? Karl, is there a use-case in which you are dealing with
`eta_bnds` without being aware of `eta`?
In the second paragraph, we currently have, "the variable names appearing
in the formula terms would differ from those found in the formula_terms
attribute of the coordinate variable itself because the 2-dimensional
bound locations do not generally coincide with the 1-dimensional
coordinate locations." I think we can state this more strongly and
clearly, "... because the boundary variables for formula terms are two-
dimensional while the formula terms themselves are one-dimensional." That
is, they must be different variables not because the locations are
different, though that it is almost certainly the case, but because they
have different dimensionality.
In the new requirement for the conformance document, can we say something
more about the dimensionality of the bounds variable of the formula terms?
I think it must have the same dimensions as the formula terms variable,
with the addition of the final (CDL) dimension of the coordinate bounds
variable.
Best wishes
Jonathan
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Ticket URL: <https://cf-trac.llnl.gov/trac/ticket/140#comment:24>
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