#147: clarification of standard and correction of conformance doc: formula_terms
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Reporter: taylor13 | Owner: cf-conventions@…
Type: defect | Status: new
Priority: high | Milestone:
Component: cf-conventions | Version:
Resolution: | Keywords:
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Comment (by taylor13):
Hi all,
There seems to be agreement (among the 4 of us who have expressed an
opinion) that attaching formula_terms to a parametric coordinate's
"boundary variable" should not raise an error (i.e., not be considered out
of conformance). I would be o.k. with either option 2 or 3 suggested by
Martin, but would favor 3: issue an information message that the attribute
has no meaning in this context. We could do this immediately. It is not
absolutely essential at this time to agree to the rest of the proposal.
[Note: I originally submitted this ticket to correct what I considered a
defect in the conformance document.]
That being said, I think we also agree that attaching formula_terms to a
"boundary variable" should be sanctioned by CF, so perhaps that also could
be implemented now. What we haven't yet reached consensus on is whether
Jonathan's alternative approach should also be allowed. As I've said I’m
against this because it would require redefinition of a "boundary
variable" in the conventions. A "boundary variable" is defined in section
1.2 ("Terminology"):
A boundary variable is associated with a variable that contains
coordinate data. When a data value provides information about conditions
in a cell occupying a region of space/time or some other dimension, the
boundary variable provides a description of cell extent.
I don’t think the parameters defined in formula terms are anything like
coordinate data; many of them are just coefficients (which do not have to
vary with location). They don’t define meaningful intervals (or cell
extents). I don’t think any compelling use case has been proposed which
would warrant complicating the very straight-forward meaning of a
“boundary variable” in the current conventions.
When we come upon a parametric coordinate variable, we might want to find
out the vertical location pointed to by that coordinate, and we would
consult the formula_terms to extract the parameters needed to do that.
Similarly, when we come upon a variable containing the bounds of a
parametric coordinate variable, we might want to find out the vertical
locations associated with these bounds, and we would consult the
formula_terms attached to it. Codes operating on the data could treat
both the coordinates and their bounds in exactly the same way. I can see
no need for an alternative pathway.
So, I would advocate further discussion before possibly implementing
Jonathan’s alternative.
In the mean time I hope this won’t hold up correcting the defect in the CF
conformance document and possibly also agreeing to use of formula_terms
with a parametric coordinate’s boundary variable.
best regards,
Karl
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Ticket URL: <https://cf-trac.llnl.gov/trac/ticket/147#comment:24>
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