#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 taylor13):
Hi David and Jonathan,
I was about to finally sign off on this when I realized what we’ve agreed
upon isn’t entirely made clear in the text. I thought Jonathan (in the
generous spirit of “reaching an agreement”) proposed that although in the
past neither explicit nor implicit methods were part of the standard, both
were used (in different datasets). For backward compatibility with these
legacy datasets, software should be able to infer the formula_terms using
either method, but going forward, we deprecate sole use of the implicit
method (but allow it in addition to the explicit method). If that is what
we’ve agreed, then would the following text be clear?
If a parametric coordinate variable with a `formula_terms` attribute
(ref section 4.3.2) also has a `bounds` attribute, its boundary variable
must have a `formula_terms` attribute too. In this case the same terms
would appear in both (as specified in Appendix D), since the
transformation from the parametric coordinate values to physical space is
realized through the same formula. For any term that depends on the
vertical dimension, however, the ''variable names'' appearing in the
formula terms would differ from those found in the `formula_terms` of the
coordinate variable itself because the 2-dimensional bound locations do
not generally coincide with the 1-dimensional coordinate locations.
Whenever a `formula_terms` attribute is attached to a boundary
variable, the formula terms may additionally be identified using a second
method: the variables appearing in the vertical coordinate's
`formula_terms` may be declared to be auxiliary coordinates, and those
coordinates may have `bounds` attributes that identify their boundary
variables. In that case, the `bounds` attribute of a formula terms
variable must be consistent with the `formula_terms` attribute of the
boundary variable. Software digesting legacy datasets (constructed prior
to version 1.7 of this standard) may have to rely in some cases on the
first method of identifying the formula term variables and in other cases,
on the second. Henceforth, however, the first method will be sufficient.
My impression is that Jonathan thought this would be o.k., but David was
worried that disallowing sole use of the “implicit” (or second) approach
might be problematic.
The "explicit" and "combined" examples provided by David in https://cf-
trac.llnl.gov/trac/ticket/140#comment:10 should also be included.
best regards,
Karl
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Ticket URL: <https://cf-trac.llnl.gov/trac/ticket/140#comment:18>
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