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20051118: navigation differences between dcgrib2 and dcgrib

David,

I'd recommend using dcgrib2, which uses the GEMPAK grib routines in
$GEMPAK/source/gemlib/gb for decoding the grib bulletins.

Dcgrib was the old program based on the unidata gribtonc grib decoder
which does not handle many of the newgrids, levels, types- so is generally
depricated. 

The dcgrib code uses the gribtonc src in $NAWIPS/unidata/ldmbridge/gribtonc_src,
and there you will find in gdes.c, the hardcoded pole point values for nmc_104.

The dcgrib2 code calculates the corner points directly in gbpolr.c, so the 
precision 
there is probably the difference in the values you see.

In the case of GRIB representation of polar stereographic, the first point
is provided, and for #104 is: -0.268000, -139.47500 which matches what you have 
for the dcgrib2 output.

Steve Chiswell
Unidata User Support



>From: David Ovens <address@hidden>
>Organization: UCAR/Unidata
>Keywords: 200511181828.jAIIS97s009471

>Hello,
>
>I have some ETA 104 grids which have been converted to GEMPAK format
>using dcgrib2 and dcgrib.  Unfortunately, there are small
>discrepancies in the navigation information between them.  Do you know
>which one is better?
>
>dcgrib2 version:
> gd_gnav rnvblk =     2.000000       1.7815592E-19    1.000000     
>    1.000000        147.0000        110.0000      -0.2680000     
>   -139.4750        32.74640       -14.59810        90.00000     
>   -105.0000        0.000000        0.000000        0.000000     
>
>dcgrib version:
> gd_gnav rnvblk =     2.000000       1.7815592E-19    1.000000     
>    1.000000        147.0000        110.0000      -0.2683270     
>   -139.4750        32.74730       -14.59930        90.00000     
>   -105.0000        0.000000        0.000000        0.000000     
>
>The entries of interest/discrepancy are 7-10 which correspond to these
>locations:
>      xlatll = rnvblk (7)  
>      xlonll = rnvblk (8)  
>      xlatur = rnvblk (9)  
>      xlonur = rnvblk (10) 
>
>Some verification code I have is producing different enough values at
>some sites because of this navigation discrepancy that I am in a
>quandary about which dcgrib to use.
>
>Thanks for any insight.
>
>David
>-- 
>David Ovens             e-mail: address@hidden
>Research Meteorologist    phone: (206) 685-8108
>Dept of Atm. Sciences      plan: Real-time MM5 forecasting for the
>Box 351640                        Pacific Northwest
>University of Washington          http://www.atmos.washington.edu/mm5rt
>Seattle, WA  98195               Weather Graphics and Loops
>                                  http://www.atmos.washington.edu/~ovens/loops
>
--
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