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/resc/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: [email protected]
[mailto:[email protected]] On Behalf Of John Caron
Sent: 15 March 2011 13:02
To: [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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