Re: [netcdf-java] problem with times in PSD dataset


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:

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:


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

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 Murray

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