Re: [CF-metadata] CF calendars (was: problem with times in PSD dataset)
On 12/14/2012 9:35 AM, Jonathan Gregory wrote: Dear Cecilia, Steve et al. Steve is right that mostly we use the Gregorian calendar. That is what I meant mostly when I said that the default is the calendar we use. The real world is mixed Julian-Gregorian, and I don't think dealing with this calendar is an issue only for Renaissance historians. I can't give you examples, but I think it is conceivable or likely that at some point people would want to record real-world data in CF earlier than the Renaissance, or have already done so. For instance, what about astronomical data, such as the dates of eclipses. These are real-world events, on precise dates which are translated into the mixed Julian-Gregorian calendar. Hi Jonathan, If scientists somewhere have encoded the dates of these historical events as data(*) using a mixed Gregorian-Julian calendar Lord help 'em. Those poor folks have to face an 11 day discontinuity in their own data, as well as in ours. I'm not meaning to be snarky. I just want to stamp out this pesky calendar issue. It has been tripping us up for too many years. Your points below are definitely the real guts of the discussion, but in this email I am addressing just the one single point. I'm afraid we will never heal ourselves from this virus if we do not eradicate it from our thinking. - Steve (*) attaching a date to an historical narrative is different from using a date as a time coordinate. It's metadata versus data. ___ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
[CF-metadata] calendar-based units of time
On 12/14/2012 10:35 AM, Jonathan Gregory wrote: I think that calendar-based units of time cannot be introduced without a new syntax for time units, and some rules about how to interpret the cases when adding months to a reference date gives an impossible date. We could make such changes to the convention. I have prototyped similar functionality in the CDM, documented here: http://www.unidata.ucar.edu/software/netcdf-java/CDM/CalendarDateTime.html John ___ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
Re: [CF-metadata] CF calendars (was: problem with times in PSD dataset)
Hi all: Heres what I understand of the conversation: 1. Theres nothing to do about existing files CF-1.6 and before. we are stuck with the udunits mixed calendar. 2. Starting with the next version after acceptance, (1.7 currently), we can do something different. I agree that forcing people to put in a calendar attribute makes simple things not simple. So lets choose a reasonable default, either gregorian_propleptic or gregorian_strict is ok with me. 3. everyone agrees that in the unit "time since date", date is interpreted in the calendar that is specified, or the default if not specified. 4. i thing we should add some advice not to start counting from 0001 when recording modern dates, no matter if you do it "right" or not. John ___ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
[CF-metadata] CF calendars (was: problem with times in PSD dataset)
Dear Cecilia, Steve et al. Steve is right that mostly we use the Gregorian calendar. That is what I meant mostly when I said that the default is the calendar we use. The real world is mixed Julian-Gregorian, and I don't think dealing with this calendar is an issue only for Renaissance historians. I can't give you examples, but I think it is conceivable or likely that at some point people would want to record real-world data in CF earlier than the Renaissance, or have already done so. For instance, what about astronomical data, such as the dates of eclipses. These are real-world events, on precise dates which are translated into the mixed Julian-Gregorian calendar. It would not be sensible to insist on translating real-world dates into the non-real-world proleptic Gregorian calendar. Hence we have to continue to support the mixed calendar. Abolishing it in CF is not one of Cecilia's four options, which only concern what the default should be. > With respect to the default calendar: > 3 replace the Julian-Gregorian calendar as default with the > proleptic Gregorian calendar I don't think this is acceptable since it changes the meaning of existing data. > 1 keep the Julian-Gregorian calendar as default (no change) Since the current default is a pitfall, changing the default would be preferable. > 2 remove the Julian-Gregorian calendar as default, and have no > default calendar (grid analogy) > 4 replace the Julian-Gregorian calendar as default with a strict > Gregorian calendar I think either of these would work. 2 causes more aggravation. It means that data which doesn't state the calendar attribute is illegal and will produce errors, even if it's entirely unproblematic such as "days since 2012-1-1". It would make CF more intolerant of existing practices than it usually has been. At the moment, CF accepts COARDS time coordinates; with this change, COARDS data would not be acceptable. Hence I would still prefer 4. The aim of this would be to make the default illegal in cases where there is a serious chance of unsafe time units, and the obvious criterion seemed to prevent dates before the invention of the Gregorian calendar. In particular that will exclude reference years of 0 and 1, which are often problematic. However, I don't feel strongly about it. As has been said in other postings, CF has its own (non-COARDS) ways of expressing climatological time. I think that calendar-based units of time cannot be introduced without a new syntax for time units, and some rules about how to interpret the cases when adding months to a reference date gives an impossible date. We could make such changes to the convention. Cheers Jonathan ___ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
