On 12/6/2012 4:09 PM, John Caron wrote:
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?
Hi Cathy,
A question to add to John's:
* If a user made a time series plot of paleo data across the
Julian-Gregorian discontinuity, what expectations would he/she have
about how the time axis was labelled? Do you know of software that
will correctly label the time axis across a mixed Julian-Gregorian
time series?
- Steve
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 thatuse 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. DonOn Wed, Dec 5, 2012 at 10:16 AM, Don Murray (NOAA Affiliate) <don.mur...@noaa.gov <mailto:don.mur...@noaa.gov>> wrote: John-I meant to send this to support-netcdf-java, but perhaps others onthe 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 thecorrect date/times.hours from 1-1-1 is rather problematic, since you are crossingthe 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 defaultcalendar is gregorian ("Mixed Gregorian/Julian calendar"). ForCDM, 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 defaultfor 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 wouldbe 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 toolsUIthan 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/> _________________________________________________ netcdf-java mailing listnetcdf-j...@unidata.ucar.edu <mailto:netcdf-j...@unidata.ucar.edu>For list information or to unsubscribe, visit: http://www.unidata.ucar.edu/__mailing_lists/ <http://www.unidata.ucar.edu/mailing_lists/>_______________________________________________ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata_______________________________________________ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
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