[gradsusr] strange behavior with GrADS 2.0.a8

[Jennifer Adams]

Hi, Don --
This one was tricky! Thank you for providing all the info I needed to  
diagnose the problem.

First of all, your grib files have 112 bytes of junk at the beginning  
of the file. One of my favorite debugging tools, the unix command 'od'  
gives me this:

 > od -c -N 116 nam.t00z.awphys01.grb2.tm00
0000000   \r  \n   -   -   4   c   7   c   8   0   6   5   3   d   e   3
0000020   \r  \n   C   o   n   t   e   n   t   -   t   y   p   e   :
0000040    t   e   x   t   /   p   l   a   i   n   ;       c   h   a   r
0000060    s   e   t   =   i   s   o   -   8   8   5   9   -   1  \r  \n
0000100    C   o   n   t   e   n   t   -   r   a   n   g   e   :       b
0000120    y   t   e   s       6   6   9   5   3   5   -   1   0   4   7
0000140    9   1   0   /   2   6   7   7   4   7   2   2  \r  \n  \r  \n
0000160    G   R   I   B

In other words, your GRIB file has the following text prefixed:

--4c7c80653de3
Content-type: text/plain; charset=iso-8859-1
Content-range: bytes 669535-1047910/26774722

Check your nam1hr.sh script and see if you can figure out how to get  
rid of that.

Secondly, as far as GrADS is concerned, the GRIB interface changed  
between a7 and a8 in order to handle files > 2Gb. This meant we had to  
stop using a routine in the GRIB2 library (seekgb) because it does not  
use 8-byte integers for file position offsets. The work normally done  
by seekgb is coded directly in the GrADS I/O layer for version a8.  
Unfortunately, one thing that seekgb does that the new code in a8 does  
not is check for junk bytes before the 4-character string "GRIB" at  
the beginning of the record. I will fix this for the new release.

In the meanwhile, try to avoid contaminating your GRIB file with junk.  
And thanks for the bug report!
--Jennifer




On Aug 31, 2010, at 1:04 AM, Don.VanDyke at noaa.gov wrote:

> Hello,
>
> I am noticing some strange behavior with GrADS 2.0.a8 when I try to  
> write a netCDF file from the following grib2 files at this link.  http://casil.met.fsu.edu/~dvandyke/GrADS-problem/ 
>   I created those grib2 files with the nam1hr.sh script at that link  
> following the example shown at http://www.cpc.noaa.gov/products/wesley/fast_downloading_grib.html 
>  .  Basically, they were created from standard NCEP 12 km NAM grib2  
> files, but they only have certain variables contained within them in  
> order to save bandwidth.  My goal is to write those variables to  
> netCDF files, but my writenc.gs example script at http://casil.met.fsu.edu/~dvandyke/GrADS-problem/writenc.gs 
>  does not work with grads2a8.  It produces an error message shown at http://casil.met.fsu.edu/~dvandyke/GrADS-problem/notes.txt 
>  .
>
> The interesting thing is that everything works perfectly using the  
> previous version of GrADS (2.0.a7.1).  However, I would like to use  
> the -flt option with the sdfwrite command, and that's only available  
> for 2.0.a8 and higher.  Can anybody explain what I'm doing wrong or  
> if this is a bug introduced with grads2a8?  Thanks!
>
> Don Van Dyke
> NWS Tallahassee, FL
> _______________________________________________
> gradsusr mailing list
> gradsusr at gradsusr.org
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--
Jennifer M. Adams
IGES/COLA
4041 Powder Mill Road, Suite 302
Calverton, MD 20705
jma at cola.iges.org



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