RE: 4D dimensions and other conventions...

• To: netcdfgroup@xxxxxxxxxxxxxxxx
• Subject: RE: 4D dimensions and other conventions...
• From: "Unidata User Support" <support@xxxxxxxxxxxxxxxx>
• Date: Mon, 28 Sep 1992 14:54:27 -0600

------- Forwarded Message

>From: HANKIN@xxxxxxxxxxxx
>Date: Mon, 28 Sep 1992 13:42:31 -0800
>Subject: RE: 4D dimensions and other conventions... 
>Organization: . 
>Keywords: 199209282044.AA08092 

>Hello Rich, Tim, Ken, et. al.,
>
>I've been following your discussion about netCDF styles with interest as my ow
> n 
>group - a numerical modeling group - shares the similar concerns:  how to use 
>netCDF to achieve a compatible representations of our model data (gridded, 
>multi-gigabyte, multiple variables on staggered grids) as well as PMEL's EPIC 
>data (down the hall) and outside institutions, too.
>
>This business of time axis representations is leading us all to similar 
>solutions. Rich has described a global variable called "base_date" which 
>"specifies the Gregorian start date".  Similarly, the file "conventions.info" 
>available from unidata.ucar.edu suggests e.g. 
>
>     variables:
>         double time(nobs);
>         time:units  = "milliseconds since (1992-9-16 10:09:55.3 -600)"
>
>Our own software, FERRET, uses a solution e.g.:
>
>        float TIME(TIME) ;
>                TIME:units = "seconds" ;
>                TIME:time_origin = "14-JAN-1976 14:00:00" ;
>
>and accepts int, long, float, or double data types.
>
>While all of these are very similar solutions they are also incompatible.  How
>  
>are time-date strings formatted?  Where should the time origin be placed: in t
> he 
>units string? in a global attribute?  in a variable attribute?  If in an 
>attribute, what is the attribute name?  Is the data type mandated?  Does the 
>axis have to be a "coordinate variable" (dimension name=variable name) ?  etc.
>  
>etc.  Similar issues arise for if/how to map gridded data onto 4-dimensional 
>grids.  Mandatory ordering of axes?  Mandatory axis names?  Mandatory units 
>choices?  What to do with missing axes (e.g. Z axis of vertically averaged 
>flow)?
>
>It seems to me that if we want to adopt conventions for these issues now is th
> e 
>time to do it.  NetCDF can fail to be a "standard" in any meaningful way if 
>these issues are not addressed somewhat formally by "users" (us) acting as a 
>community.  I have some personal experiences with this type of standards-failu
> re 
>as a member of the ANSI committee that creates CGM (the Computer Graphics 
>Metafile).  CGM, a broadly conceived standard, has expected user communities t
> o 
>develop "profiles" that dictate their particular style choices and ensure 
>interoperability.  The user communities have mostly failed to get organized an
> d 
>there is chaos in the CGM world - enough to endanger its success as a standard
> .
>
>I spoke to Russ Rew and he agreed that a "straw man" proposal on these 
>conventions for oceanographers was in order.  I will try to pull one together 
> in 
>the next few days - using "conventions.info" as a starting point but going int
> o 
>much greater detail.  My main goal will be to enumerate the open issues.  The 
>list I generate will be VERY incomplete - I hope we can pass it around and add
>  
>to it.  When we have a moderately exhaustive list then we can begin discussing
>  
>solutions that encompass our issues.
>
>If you see a problem with this process please fire away!
>
>       cheers - steve
>

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