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Re: Reading GRIB2 data

On Tue, 20 May 2008, Ethan Davis wrote:


Hi Robb,

A few questions on GRIB2 and reading it through the netCDF-Java library:

1) Is each GRIB record individually compressed?


yes
So, each level of each field 
is separately compressed?
yes, so if a parm has 30 levels it requires 30 different data reads to get all the levels
2) What information is in the index files? Parameter id, levels, grid definition, and of course the location in the file? The reason I'm asking is I'm wondering if there are any times the actual GRIB file has to be opened other than when data values are requested? 

Section 2 Grib records
one line for each record in the file
blank separated
ProductType Discipline Category Parameter TypeGenProcess LevelType1 LevelValue1 LevelType2 LevelValue2 ReferenceTime ForecastTime Gdskey GdsOffset PdsOffset 

noop, the only time the file needs to be opened is when data is requested.
3) Once a GRIB2 dataset is opened and data is requested for one level of one parameter, does that uncompressed data get cached in-case other requests for the same level and parameter are made?
the index is cached but i don't think the data is cached because it's large. John handles that part 




4) Are the decompressed records in memory or on disk?
they are represented by offsets to a RAF, there is some caching by the RAF. 

robb...


That's it for now.

Thanks,

Ethan

--
Ethan R. Davis                                Telephone: (303) 497-8155
Software Engineer                             Fax:       (303) 497-8690
UCAR Unidata Program Center                   E-mail:    address@hidden
P.O. Box 3000
Boulder, CO  80307-3000                       http://www.unidata.ucar.edu/
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Robb Kambic                                Unidata Program Center
Software Engineer III                      Univ. Corp for Atmospheric Research
address@hidden             WWW: http://www.unidata.ucar.edu/
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