[CF-metadata] udunits handling of fuzzy time units

From: Dave Allured <dave.allured>
Date: Mon, 14 Mar 2011 21:40:46 -0600

There are several existing solutions to your question, depending on
the constraints or requirements of your application. Some of these
use encodings that are not supported by Udunits, but are explicitly
legal under the current version of CF. Here are two leading solutions:

1. time at units = "days since 1930-01-01"
+ time at calendar = "gregorian"

* Compatible with current CF conventions
* Compatible with Udunits-1 and -2
* Compatible with many existing general purpose softwares
* Conforms to the "western world's" actual historic calendar
* Complies with CF and COARDS precautions to AVOID fuzzy
. units of months or years
* Numeric time step is NOT constant for increments of
. months or years

2. time at units = "months (or years) since 1930-01-01"
+ time at calendar = "360_day"

* Compatible with current CF conventions
* NOT compatible with Udunits-1 or -2
* Compatible with SOME existing general purpose softwares
* DOES NOT conform to the "western world's" actual historic calendar
* DOES NOT comply with CF and COARDS precautions to AVOID fuzzy
. units of months or years, but reasonable in this context
* Numeric time step IS constant for increments of
. months or years

So, what constraints are important for you, and will either of the
above solutions be satisfactory?

--Dave A.
NOAA/PSD/CIRES

On 3/14/2011 6:31 PM, John Caron wrote:
> because udunits converts units like months and years to a fixed
> number of seconds, one cant really use things like months and years
> as units, since you get things like this:
>
> 0 months since 1930-01-01 == 1930-01-01T00:00:00Z
> 1 months since 1930-01-01 == 1930-01-31T10:29:03Z
> 2 months since 1930-01-01 == 1930-03-02T20:58:07Z
> 3 months since 1930-01-01 == 1930-04-02T07:27:11Z
> 4 months since 1930-01-01 == 1930-05-02T17:56:15Z
> 5 months since 1930-01-01 == 1930-06-02T04:25:19Z
> 6 months since 1930-01-01 == 1930-07-02T14:54:22Z
> 7 months since 1930-01-01 == 1930-08-02T01:23:26Z
> 8 months since 1930-01-01 == 1930-09-01T11:52:30Z
> 9 months since 1930-01-01 == 1930-10-01T22:21:34Z
> 10 months since 1930-01-01 == 1930-11-01T08:50:38Z
> 11 months since 1930-01-01 == 1930-12-01T19:19:42Z
>
> 0 years since 1850-01-01 == 1850-01-01T00:00:00Z
> 10 years since 1850-01-01 == 1860-01-01T10:07:39Z
> 20 years since 1850-01-01 == 1869-12-31T20:15:19Z
> 30 years since 1850-01-01 == 1880-01-01T06:22:59Z
> 40 years since 1850-01-01 == 1889-12-31T16:30:38Z
> 50 years since 1850-01-01 == 1900-01-01T02:38:18Z
> 60 years since 1850-01-01 == 1910-01-01T12:45:58Z
> 70 years since 1850-01-01 == 1920-01-01T22:53:38Z
> 80 years since 1850-01-01 == 1930-01-01T09:01:17Z
> 90 years since 1850-01-01 == 1940-01-01T19:08:57Z
>
> whereas you might really want:
>
> 0 months since 1930-01-01 = 1930-01-01T00:00Z
> 1 months since 1930-01-01 = 1930-02-01T00:00Z
> 2 months since 1930-01-01 = 1930-03-01T00:00Z
> 3 months since 1930-01-01 = 1930-04-01T00:00Z
> 4 months since 1930-01-01 = 1930-05-01T00:00Z
> 5 months since 1930-01-01 = 1930-06-01T00:00Z
> 6 months since 1930-01-01 = 1930-07-01T00:00Z
> 7 months since 1930-01-01 = 1930-08-01T00:00Z
> 8 months since 1930-01-01 = 1930-09-01T00:00Z
> 9 months since 1930-01-01 = 1930-10-01T00:00Z
> 10 months since 1930-01-01 = 1930-11-01T00:00Z
> 11 months since 1930-01-01 = 1930-12-01T00:00Z
>
> 0 years since 1850-01-01 = 1850-01-01T00:00Z
> 10 years since 1850-01-01 = 1860-01-01T00:00Z
> 20 years since 1850-01-01 = 1870-01-01T00:00Z
> 30 years since 1850-01-01 = 1880-01-01T00:00Z
> 40 years since 1850-01-01 = 1890-01-01T00:00Z
> 50 years since 1850-01-01 = 1900-01-01T00:00Z
> 60 years since 1850-01-01 = 1910-01-01T00:00Z
> 70 years since 1850-01-01 = 1920-01-01T00:00Z
> 80 years since 1850-01-01 = 1930-01-01T00:00Z
> 90 years since 1850-01-01 = 1940-01-01T00:00Z
>
> the solutions that occur to me are:
>
> 1) use ISO date strings
> 2) change the way udunits works
> 3) create another syntax that can be distinguished from udunits and
> make it work like the above
> 4) something else i havent thought of
>
> any ideas?
Received on Mon Mar 14 2011 - 21:40:46 GMT