Hi Cathy:
There no question that CF currently defaults to mixed gregorian
calendar. The discussion is whether thats the best choice (probably
not), and to advise users not to cross the discontinuity (eg store
modern dates starting from 1-1-1).
Im curious as to how you generate the dates that you store? That is, how
do you know that they are correct?
John
On 12/6/2012 4:34 PM, Cathy Smith (NOAA Affiliate) wrote:
> John
> There is some meteorological data that is available pre-Gregorian
> calendar (paleo data, some temperature datasets) and of course there are
> other scientific fields where data is pre-1500 (e.g. astronomy,
> archeology) . Given that netCDF files with data dates spanning ~1550 probably already exist and the large number of preexisting files that
> use the 1-1-1 base (we have over 2000), it doesn't seem reasonable to
> request that files be changed to accommodate what is essentially a bug;
> the date values we store are correct. We started using the 1-1-1 base in the mid
> 1990's (almost 20 years ago) as part of the COARDS (now CF) agreed upon standard.
> It is reasonable for us to consider future changes but I don't think it reasonable for us to have to change our files because the Java interface is not backwards compatible.
>
> Cathy Smith
> NOAA/ESRL PSD
>
> On 12/5/12 12:25 PM, John Caron wrote:
>> Hi all:
>>
>> Its probably the right thing to do to make gregorian ("Mixed
>> Gregorian/Julian calendar") the default calendar for COARDS/CF, for
>> backwards compatibility. However, CDM may leave proleptic_gregorian
>> (ISO8601) as its default.
>>
>> And I would strongly suggest that data writers stop using "time since
>> 1-1-1". Ive never seen a dataset where "time since 1-1-1" using the
>> mixed gregorian calendar was actually needed. If any one has a real
>> example, Id like to hear about it.
>>
>> If you really need "historical accuracy", then put in an ISO8601
>> formatted string, and an explicit calendar attribute. CDM handles
>> those ok. CF should be upgraded to allow ISO strings also. "time since
>> reference date" is not good for very large ranges of time.
>>
>> Ill just add that udunits never wanted to be a calendaring library,
>> and shouldnt be used anymore for that. Im relying on joda-time (and
>> its successor threeten) to be the expert in calendering, so i dont
>> have to. I think the netcdf-C library now uses some CDAT (?) code for
>> its calendaring, but Im sure theres other standard libraries that
>> could be used. Anyone have candidate libraries in C or Python for
>> robust calendering>
>>
>> In short, we should rely on clear encoding standards (eg ISO8601) with
>> reference software, rather than implementations like udunits that
>> eventually go away.
>>
>> John
>>
>> PS: I think ill cross post to cf, just to throw some gasoline on the
>> fire ;), and maybe some broader viewpoints.
>>
>> On 12/5/2012 10:24 AM, Don Murray (NOAA Affiliate) wrote:
>>> Hi Gerry-
>>>
>>> On 12/5/12 9:42 AM, Gerry Creager - NOAA Affiliate wrote:
>>>> There are other datasets with reference to 1-1-1. I've seen them most
>>>> recently in some ocean models.
>>>
>>> And the ESRL/PSD NCEP reanalysis datasets use it.
>>>
>>> Don
>>>
>>>> On Wed, Dec 5, 2012 at 10:16 AM, Don Murray (NOAA Affiliate)
>>>> <don.murray at noaa.gov <mailto:don.murray at noaa.gov>> wrote:
>>>>
>>>> John-
>>>>
>>>> I meant to send this to support-netcdf-java, but perhaps others on
>>>> the list might have the same problem.
>>>>
>>>>
>>>> On 12/4/12 4:51 PM, John Caron wrote:
>>>>
>>>> On 12/4/2012 4:09 PM, Don Murray (NOAA Affiliate) wrote:
>>>>
>>>> Hi-
>>>>
>>>> I was just trying to access the NOAA/ESRL/PSD Outgoing
>>>> Longwave
>>>> Radiation (OLR) data using netCDF-Java 4.3 ToolsUI and
>>>> noticed that the
>>>> times are wrong. If you open:
>>>>
>>>>
>>>> dods://www.esrl.noaa.gov/psd/__thredds/dodsC/Datasets/__uninterp_OLR/olr.day.mean.nc
>>>>
>>>>
>>>>
>>>> <http://www.esrl.noaa.gov/psd/thredds/dodsC/Datasets/uninterp_OLR/olr.day.mean.nc>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> in the ToolsUI grid viewer, the last time in the file is
>>>> shown as
>>>> 2012-12-04 00Z. However, the last time in the file is
>>>> actually
>>>> 2012-12-02 00Z. Here is the time variable in that file:
>>>>
>>>> double time(time=3989);
>>>> :units = "hours since 1-1-1 00:00:0.0";
>>>> :long_name = "Time";
>>>> :actual_range = 1.7540448E7, 1.763616E7; // double
>>>> :delta_t = "0000-00-01 00:00:00";
>>>> :avg_period = "0000-00-01 00:00:00";
>>>> :standard_name = "time";
>>>> :axis = "T";
>>>>
>>>> netCDF-Java 4.2 and ncdump -t -v time (C version) show the
>>>> correct
>>>> date/times.
>>>>
>>>>
>>>> hours from 1-1-1 is rather problematic, since you are crossing
>>>> the
>>>> julian/gregorian weirdness line (i think thats the technical
>>>> term ;)
>>>>
>>>> Im guessing the trouble lies here:
>>>>
>>>> "Default calendar: for udunits, and therefore for CF, the
>>>> default
>>>> calendar is gregorian ("Mixed Gregorian/Julian calendar"). For
>>>> CDM, the
>>>> default calendar is proleptic_gregorian (ISO8601 standard).
>>>> This
>>>> only
>>>> matters for dates before 1582."
>>>>
>>>>
>>>> Joda time supports the GJ calendar (Historically accurate calendar
>>>> with Julian followed by Gregorian) which seems it would be backward
>>>> compatible with the CF/udunits. Perhaps that should be the default
>>>> for backward compatibility.
>>>>
>>>>
>>>> I have to say relying uncritically on a calendar
>>>> implementation like
>>>> udunits is a mistake. putting the reference date
>>>> unnecessarily to
>>>> include the problem is, um, unnecessary.
>>>>
>>>>
>>>> But it is historically accurate. For climate datasets, this would
>>>> be important.
>>>>
>>>>
>>>> is there any way those files can be updated? specifying the
>>>> gregorian
>>>> calendar explicitly should do it, but changing to use a
>>>> reference date
>>>> after 1582 would be much better.
>>>>
>>>>
>>>> How's your FORTRAN? ;-) I'm not sure why this was chosen
>>>> originally, but it doesn't seem reasonable to make people change
>>>> their datasets.
>>>>
>>>> Does anyone else on the list know of datasets (other than
>>>> climatologies) that might use a reference of 1-1-1 that will be
>>>> affected by this change?
>>>>
>>>>
>>>>
>>>> BTW, is there an easier way to see human readable dates
>>>> through toolsUI
>>>> than loading it into the grid viewer (akin to ncdump -t)?
>>>>
>>>>
>>>> open in coordSys tab; in bottom table, select time coord,
>>>> right-click
>>>> and choose "show values as date"
>>>>
>>>>
>>>> Thanks, that's easier.
>>>>
>>>>
>>>> Don
>>>> --
>>>> Don Murray
>>>> NOAA/ESRL/PSD and CIRES
>>>> 303-497-3596 <tel:303-497-3596>
>>>> http://www.esrl.noaa.gov/psd/__people/don.murray/
>>>> <http://www.esrl.noaa.gov/psd/people/don.murray/>
>>>>
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>>>>
>>>
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Received on Thu Dec 06 2012 - 17:09:14 GMT
