[ncl-talk] incorrect date using cd_calendar (and cd_string)?

[Dave Allured - NOAA Affiliate]

Dennis,

Ah, thanks for finding the actual problem.  However, in CF section 2.6,
"Attributes", VALUES of character attributes have a different rule than
attribute NAMES:

    "... When this standard defines string attributes that may take
    various prescribed values, the possible values are generally
    given in lower case. However, applications programs should
    not be sensitive to case in these attributes ..."

So "JULIAN" and "julian" should both be correct, and the file is CF
compliant.  Regardless, the best interim fix for Denise is to convert the
calendar attribute to lowercase, before using date functions like
cd_calendar.

I agree that attribute NAMES such as "units" and "calendar" must be lower
case according to CF.

--Dave


On Wed, Nov 15, 2017 at 5:59 PM, Dennis Shea <shea at ucar.edu> wrote:

> Ummm,  I don't this is a bug.
>
> I think the file does not adhere to the Climate and Forecast netCDF
> convention
>
> [1] CF Convention: calendar attribute
>
>      http://cfconventions.org/Data/cf-conventions/cf-conventions-
> 1.6/build/cf-conventions.html#calendar
>
> [2] The recognized *calendar *attributes are all lower case. No mention
> is made of 'case insensitivity.'
>      As an example: the *units *attribute is lower case; UNITS, Units etc
> are **not* *recognized.
>
> [3] See attached script.
>
> %> ncl test_time_julian.ncl
>
> ------
>
> Variable: C
> Type: float
> Total Size: 24 bytes
>             6 values
> Number of Dimensions: 2
> Dimensions and sizes:    [1] x [6]
> Coordinates:
> Number Of Attributes: 1
>
>
>
>
> *calendar :    JULIAN(0,0)    2015(0,1)     5(0,2)    13(0,3)    12*
> (0,4)     0
> (0,5)     0
> (0)    =========
>
>
> Variable: c
> Type: float
> Total Size: 24 bytes
>             6 values
> Number of Dimensions: 2
> Dimensions and sizes:    [1] x [6]
> Coordinates:
> Number Of Attributes: 1
>
>
>
>
> *calendar :    julian(0,0)    2015(0,1)     4(0,2)    30(0,3)    12*
> (0,4)     0
> (0,5)     0
>
> The latter matches ferret and (more important) DaveA's hand calculation.
>
>
> On Wed, Nov 15, 2017 at 3:21 PM, Dave Allured - NOAA Affiliate <
> dave.allured at noaa.gov> wrote:
>
>> Denise,
>>
>> Your sample file uses the Julian calendar.  In my experience the Julian
>> calendar is rarely used in climate sciences, and software implementations
>> are not well vetted.  The best long term solution might be to switch to the
>> Gregorian calendar, if that is not contrary to your current application.
>>
>> Also your time base is "hours since 0001-01-01 00:00:00".  Using a base
>> date before the Julian to Gregorian change in year 1582 exposes you to a
>> particular kind of software problem.  You might get better results if you
>> can switch to a base date in the near past, such as 1900-1-1 or 2000-1-1,
>> regardless of which calendar system is selected.
>>
>> For what it's worth, my hand calculation for Julian 17657580 hours since
>> 1-1-1 0:0:0 gets 2015 April 30 12:00.  This agrees with your ferret result.
>>
>> --Dave
>>
>>
>> On Wed, Nov 15, 2017 at 1:00 PM, Denise Worthen - NOAA Affiliate <
>> denise.worthen at noaa.gov> wrote:
>>
>>> Hi,
>>>
>>> I've used ncra to 'record average' a set of netcdf files. When I look at
>>> the file using ferret, the date in each file is correctly interpreted.
>>>
>>> When I use ncl, the dates in the file are interpreted incorrectly,
>>> either as a label (using cd_string) or just printing the value using
>>> cd_calendar.
>>>
>>> I've extracted out the time axis for a single timestep and attached it
>>> here. The correct value in this case should be april 30 2015.
>>>
>>> Using the following commands, it shows the date as 5-13-2015. This is
>>> also the label I get when using cd_string to label a plot in the original
>>> file.
>>>
>>> What have I got wrong?
>>> Denise.
>>>
>>>
>>> load "$NCARG_ROOT/lib/ncarg/nclscripts/contrib/cd_string.ncl"
>>>
>>> begin
>>>
>>> f=addfile("timetest.nc","r")
>>> timeval=f->time
>>> timeUnits = f->time at units
>>> timeCalendar = f->time at calendar
>>> print(cd_calendar(timeval,0))
>>> end
>>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.ucar.edu/pipermail/ncl-talk/attachments/20171115/6146456e/attachment.html>