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20041025: Level-2 data from NCDC and compression

>From: Chris Herbster <address@hidden>
>Organization: Embry-Riddle
>Keywords: 200410252112.i9PLCQvV019645 Level II

Hi Chris-

>I have downloaded some level-2 data from the NCDC archive and was 
>wondering if there is a "reverse- decoder" (wouldn't that be an 
>"encoder") to compress the data to save on disk space?  The files come 
>from NCDC in the Unix compress (*.Z) format. 

We don't have one.

>Can I just "bzip" the file, or does the file have the compression of the 
>data internal to the file format? 

You can't just bzip the file because then you'd be compressing the
24 byte header.  For the bzip2 stuff on the LDM feed, it consists
of an uncompressed header, folllowed by a block with a byte count
of the compressed data and then the compressed data.  So, you could
write a program to take the first 24 bytes, write that to a new
file, then compress the rest of the data, write out the number of
compressed bytes, then the compressed data.  It would look like:

|header(24bytes)|numCompressedBytes(int)|compressed data

Alternatively, you could do multiple smaller chunks, being sure that
the first word is the number of compressed bytes following.  If
you do create such a program, it would be a good contribution to
the community.

In theory, both GEMPAK and IDV should be able to read this format.

>We are mostly using the IDV to look at the data, but we also expect to 
>use the gempak programs that are available.

Either should read the uncompressed data or something akin to 
the compression scheme above.

>Finally, do you know if anyone is working on a gdradr version that could 
>generate a 3-D mosaic?

This would be quite a challenge.  I guess people in Oklahoma are doing
something like this for their assimilation schemes, but I'm not 
sure what they are doing.

Don Murray
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