Re: I solved the Nexrad mystery! (was: Can rad handle new stuff...)

I have also been able to uncompress the ARX NIDS data but I needed to do some 
tricks to get it to work.  This is what I did... its pretty ugly!!

It turns out that NWS has split the NIDS file into 4000 byte blocks and then 
compressed the data using the zlib compress utility.  Then the compressed 
blocks are sent out sequentially as part of the whole product.  This means 
you have separate the blocks to decode.  

The problem is that the uncompress utility doesn't work that effectively.  
The problem is you don't know the size of the compressed blocks.  The block 
sizes are buried in the SBN header which is masked off by the LDM.  So you 
have to guess what the block sizes are.  The uncompress utility will return 
the size of the uncompressed data but won't reveal the size of the compressed 
data. 

What I've done is to forward through the data 1000 bytes at a time and then 
try to uncompress the data.  If the function returns an error, I increment 
through the data a byte at a time until I get a valid return on the 
uncompression.  This is just plain ugly....

What I'm looking for is a simpler solution!!  If there is another utility 
that gives me the compressed block sizes, this would make my job much easier!

BTW, I read where the uncompress utility will uncompress all the data if the 
file is memory mapped???  Otherwise, it only works a block at a time.

So Pete, can you forward the Perl script so I can check what functions are 
used??

Dan.... from cloudy Chicago!!