On 3/15/2011 9:19 AM, Benno Blumenthal wrote:
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.
i think the issue is that udunits is not a good reference library for
date handling, and that we should create a new reference library.
the problem with udunits is:
1) it does not support calenders.
2) it allows units of month and year, but implements them in a way
thats appropriate to dimensional units, not calendar dates.
Benno
On Tue, Mar 15, 2011 at 9:14 AM, Jon Blower <[email protected]
<mailto:[email protected]>> 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: [email protected]
<mailto:[email protected]>
[mailto:[email protected]
<mailto:[email protected]>] On Behalf Of John Caron
Sent: 15 March 2011 13:02
To: [email protected] <mailto:[email protected]>
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 [email protected]
<mailto:[email protected]>
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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