Dear All,
At the nit-picking level, "day" (and "hour" and "minute") are not
necessarily stable units either, because of the occasional appearance
of leap seconds. While this won't be of much concern for many users,
it can be important for precisely timed data.
Tim.
On 15/03/2011 16:24, "John Caron" <caron at unidata.ucar.edu> wrote:
>On 3/15/2011 7:14 AM, Jon Blower 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.
>
>yes, some of the confusion is that "time" _is_ a unit of measure and
>"month" and "year" are well defined in that context. As long as your
>units are dimensionally equivalent to "secs" then theres no problem.
>
>the problem is that "date" is not a dimensional unit, its a different
>animal. And the "time coordinate" is a date. And dates ought to
>correspond to our usual meaning of date, but "month" and "year" skewer
>that.
>
>> Since CF defines that we use the UDUNITS interpretation of month/year,
>>it would seem dangerous to change this assumption for backward
>>compatibility?
>
>We could handle backwards compatibility with the version number of CF.
>
>> (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/res
>>c/edal/time.
>
>thanks, ill check that out
>
>> I even wrote half-decent unit tests - aren't I a good boy? ;-)
>
>the Overlords will be pleased when They arrive ;^)
>
>> Jon
>>
>> -----Original Message-----
>> From: 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
>> 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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Received on Tue Mar 15 2011 - 13:30:34 GMT
