[netcdfgroup] Setting special attributes using ncgen

• To: NetCDF Group List <netcdfgroup@xxxxxxxxxxxxxxxx>
• Subject: [netcdfgroup] Setting special attributes using ncgen
• From: Roy Mendelssohn <roy.mendelssohn@xxxxxxxx>
• Date: Sat, 7 Apr 2012 16:55:12 -0700

Hi All:

I am using ncgen (version 4) to create a file.  Just out out of the box, it 
creates a netcdf3 file much larger than I care to work with, so i want to 
create the netcf4 with even default compression is fine.  "man ncgen" tells me:

Special Attributes
Special, virtual, attributes can be specified to provide performance-related 
information about the file format and about variable properties. The file must 
be a netCDF-4 file for these to take effect.
These special virtual attributes are not actually part of the file, they are 
merely a convenient way to set miscellaneous properties of the data in CDL
The special attributes currently supported are as follows: ‘_Format’, 
‘_Fletcher32, ‘_ChunkSizes’, ‘_Endianness’, ‘_DeflateLevel’, ‘_Shuffle’, and 
‘_Storage’.
‘_Format’ is a global attribute specifying the netCDF format variant. Its value 
must be a single string matching one of ‘classic’, ‘64-bit offset’, ‘netCDF-4’, 
or ‘netCDF-4 classic model’.
The rest of the special attributes are all variable attributes. Essentially all 
of then map to some corresponding ‘nc_def_var_XXX’ function as defined in the 
netCDF-4 API. For the atttributes that are essentially boolean (_Fletcher32, 
_Shuffle, and _NOFILL), the value true can be specified by using the strings 
‘true’ or ‘1’, or by using the integer 1. The value false expects either 
‘false’, ‘0’, or the integer 0. The actions associated with these attributes 
are as follows.
1.
‘_Fletcher32 sets the ‘fletcher32’ property for a variable.
2.
‘_Endianness’ is either ‘little’ or ‘big’, depending on how the variable is 
stored when first written.
3.
‘_DeflateLevel’ is an integer between 0 and 9 inclusive if compression has been 
specified for the variable.
4.
‘_Shuffle’ specifies if the the shuffle filter should be used.
5.
‘_Storage’ is ‘contiguous’ or ‘chunked’.
6.
‘_ChunkSizes’ is a list of chunk sizes for each dimension of the variable

**********************
I know very little about any of this, and some examples would have been 
helpful.  my dimensions are (time, altitude, latitude, longitude)  and example 
for one variable with the se dimensions to achieve a reasonable amount of 
compression would be of great help.

Thanks,

-Roy



"The contents of this message do not reflect any position of the U.S. 
Government or NOAA."
**********************
Roy Mendelssohn
Supervisory Operations Research Analyst
NOAA/NMFS
Environmental Research Division
Southwest Fisheries Science Center
1352 Lighthouse Avenue
Pacific Grove, CA 93950-2097

e-mail: Roy.Mendelssohn@xxxxxxxx (Note new e-mail address)
voice: (831)-648-9029
fax: (831)-648-8440
www: http://www.pfeg.noaa.gov/

"Old age and treachery will overcome youth and skill."
"From those who have been given much, much will be expected" 
"the arc of the moral universe is long, but it bends toward justice" -MLK Jr.

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  • Re: [netcdfgroup] Setting special attributes using ncgen
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