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.mur...@noaa.gov <mailto:don.mur...@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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