[CF-metadata] udunits handling of fuzzy time units

From: John Caron <caron>
Date: Tue, 15 Mar 2011 06:50:34 -0600

On 3/14/2011 9:40 PM, Dave Allured wrote:
> 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
Hi Dave:

option 1 is suboptimal because one has to calculate the days correctly.
Also it makes the time coordinates not human readable, eg:

   time:units = "days since 1850-01-01";

   values: 0, 365, 730, 1096, 1461, 1826, 2191, 2557, 2922, 3287, 3652,
4018, 4383, 4748, 5113, 5479, 5844, 6209, 6574, 6940, 7305, 7670, 8035,
8401, 8766, 9131, 9496, 9862, 10227, 10592, 10957, 11323, 11688, 12053,
12418, 12784, 13149, 13514, 13879, 14245, 14610, 14975, 15340, 15706,
16071, 16436, 16801, 17167, 17532, 17897, 18262, 18627, 18992, 19357,
19722, 20088, 20453, 20818, 21183, 21549, 21914, 22279, 22644, 23010,
23375, 23740, 24105, 24471, 24836, 25201, 25566, 25932, 26297, 26662,
27027, 27393, 27758, 28123, 28488, 28854, 29219, 29584, 29949, 30315,
30680, 31045, 31410, 31776, 32141, 32506, 32871, 33237, 33602, 33967,
34332, 34698, 35063, 35428, 35793, 36159;

vs

time:units = "years since 1850-01-01";

   values: 0, 1, 2, 3 etc;


option 2, i am working on adding non-standard calenders to the CDM, so I
will look at that closer.

thanks
Received on Tue Mar 15 2011 - 06:50:34 GMT