Hi Richard,
I have some opinions and discussion on some of your points.
On 7/1/2013 8:41 AM, Steve Hankin wrote:
> Yo may want to follow/participate on this discussion
>
>
> -------- Original Message --------
> Subject: [CF-metadata] Non-real-world calendars
> Date: Mon, 1 Jul 2013 13:26:22 +0000
> From: Hattersley, Richard <richard.hattersley at metoffice.gov.uk>
> To: cf-metadata at cgd.ucar.edu <cf-metadata at cgd.ucar.edu>
>
>
>
> Hi everyone,
> I'd like to propose a trac ticket or two to clarify the meaning when
> using alternative calendars. But before I do that I'd like to check
> for community opinion (or even consensus!?) ...
>
For one thing, there should be a definition of "real world calendars",
shouldn't there.
> 1. Time zones should be excluded/banned when using non-real-world
> calendars. For example, the statement in section 4.4 of "if the time
> zone is omitted the default is UTC" should not apply.
>
> 2. The "months since" and "years since" semantics for non-real-world
> calendars need defining/outlawing. e.g. The UDUNITS definition of a
> year as 365.242198781 days makes no sense at all for a 360-day
> calendar, but in this particular case a year could be unambiguously
> defined as 360 days.
>
For each calendar, there's a year length in days. The definition of
"years-since", etc. would always flow from the calendar definition
that's in place. So if the definition of time is noleap (equivalent to
Udunits common_year), then years_since or days-since would be computed
using a 365-day year. Is that what the Udunits library does? "Month"
is inherently ambiguous as discussed in the CF document, but would be
1/12 of a year. In CF, the definition of a year as 365.242198781 days
doesn't apply to any of the calendars, because it doesn't relate to
calendar months/days/hours etc. (Does the Udunits library use that
number? How?)
All of the calendars in CF section 4.4.1 have definitions that allow
software to convert between time coordinates and date-strings using the
unit, time origin and calendar. They're consistent within themselves.
Each one implies a number of days/fractional days per year.
Ansley
> 3. The year-zero semantics for non-real-world calendars need defining.
> From section 7.4, "Year 0 may be a valid year in non-real-world
> calendars".
> I have some further questions concerning real-world calendars, but as
> with all things dealing with the real world they are a little more
> messy so I'll save them for another post.
> Richard Hattersley
> Benevolent Dictator of Iris - a CF library for Python:
> www.scitools.org.uk/iris <http://www.scitools.org.uk/iris>
> *Met Office* FitzRoy Road Exeter Devon EX1 3PB United Kingdom
> Tel: +44 (0)1392 885702
> Email: richard.hattersley at metoffice.gov.uk
> <mailto:richard.hattersley at metoffice.gov.uk> Web:
> www.metoffice.gov.uk <http://www.metoffice.gov.uk/>
>
>
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Received on Tue Jul 02 2013 - 10:49:24 BST
