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[CF-metadata] udunits handling of fuzzy time units

From: Jon Blower <j.d.blower>
Date: Tue, 15 Mar 2011 16:45:13 +0000

Benno,

So are you suggesting that CF should allow "years since" for 360, 365, and 366-day calendars and "months since" for 360 day calendars (and continue to deprecate both of these for other systems)?

If so, I can see how this would be convenient, but it means that effectively we're not using UDUNITS to define the unit string in these special cases.

I can also see the benefit in disassociating the time units string from UDUNITS, which was one of John's suggestions, but I would worry about backward compatibility (haven't thought this through).

Jon

From: bennoblumenthal at gmail.com [mailto:bennoblumenthal at gmail.com] On Behalf Of Benno Blumenthal
Sent: 15 March 2011 15:20
To: Jon Blower
Cc: John Caron; cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] udunits handling of fuzzy time units

I am sorry, but this conversation is more confusing that it needs to be -- once calendar 360_day is chosen, there is nothing "fuzzy" about month or year, and once calendar 365_day or 366_day is chosen, there is nothing "fuzzy" about year. udunits does not support calendar, so its poor choice of month/year support is not an issue -- if it did support calendar (which is in the standard), then it would handle year/month correctly for these choices of calendar.

Benno
On Tue, Mar 15, 2011 at 9:14 AM, Jon Blower <j.d.blower at reading.ac.uk<mailto:j.d.blower at reading.ac.uk>> wrote:
I think it's good to remove the dependence on UDUNITS from the CDM for date handling.

However, although "date" is not a unit of measure, "seconds" is, and so is "month" in the definition of UDUNITS. Since CF defines that we use the UDUNITS interpretation of month/year, it would seem dangerous to change this assumption for backward compatibility?

(It's not just that months are of variable lengths within a year, but also that there are different definitions of a "month". UDUNITS uses a fixed year-length (not a calendar year length) and a month is year/12.)

BTW, the various calendars are implemented in ncWMS at http://www.resc.rdg.ac.uk/trac/ncWMS/browser/trunk/src/java/uk/ac/rdg/resc/edal/time.

I even wrote half-decent unit tests - aren't I a good boy? ;-)

Jon

-----Original Message-----
From: cf-metadata-bounces at cgd.ucar.edu<mailto:cf-metadata-bounces at cgd.ucar.edu> [mailto:cf-metadata-bounces at cgd.ucar.edu<mailto:cf-metadata-bounces at cgd.ucar.edu>] On Behalf Of John Caron
Sent: 15 March 2011 13:02
To: cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>
Subject: Re: [CF-metadata] udunits handling of fuzzy time units

On 3/15/2011 5:03 AM, Karl Taylor wrote:
> I agree with Jon.
>
> By definition, I think, a "unit of measure" must not vary; hence month
> is not a proper unit and not only depends on month of year, but also
> on assumed calendar (and similarly for year). Therefore, I think
> "months since" and "years since" should not be allowed in CF.
>
> Karl

Hi Karl:

so if currently we cant actually use months and years, because of the
way udunits handles them, why not redefine how they should be understood
when you do use them, namely as setting the month or year field in a
date calculation.

this eases the burden on data writers, and makes the metadata human
readable, at the cost of a small increase in the complexity of libraries
that read data.

one more comment: a date is not a unit of measure, and therein lies all
the trouble. IMO, date handling should be removed from the udunits
package, which is what im doing now in the CDM (not removing date
handling from udunits, just not using udunits anymore to handle dates).

John
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--
Dr. M. Benno Blumenthal          benno at iri.columbia.edu<mailto:benno at iri.columbia.edu>
International Research Institute for climate and society
The Earth Institute at Columbia University
Lamont Campus, Palisades NY 10964-8000   (845) 680-4450
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