navigation information in GRIB2 datasets on THREDDS server
John Caron
caron at unidata.ucar.edu
Tue Sep 5 15:21:05 MDT 2006
We are looking at adding those parameters, ill let you know.
David Wojtowicz wrote:
> I see, however I have a significantly large pre-existing code base that uses
> the GRIB navigation parameters for everything.
>
> This code originally read GRIB1 files directly. I had previously adapted it
> to read data from the THREDDS server that used the NUWG conventions, and it
> was relatively straightforward as the GRIB1 nav params were passed through
> fairly directly in the various attributes.
>
> Now however, I'm stuck in modifying it to deal with the GRIB2 based datasets
> in THREDDS. I'm not familiar enough with the navigation systems to make the
> translations that would be necessary.
>
> Would it be possible to pass the GRIB2 navigation params through as
> ancillary information in the new convention? Or even just copy the nav block
> (GRIB2 section 3) octets as an integer array into the dataset? (or at least
> point me to the place where the params are converted from GRIB to FGDC so I
> could attempt to reverse the translation) Redoing the code I have to use
> FGDC navigation would be a huge undertaking.
>
>
> -----
> David Wojtowicz, Sr. Research Programmer, Sysadmin
> Dept of Atmospheric Sciences / Computer Services
> University of Illinois at Urbana-Champaign
> davidw at uiuc.edu (217) 333-8390
>
> -----Original Message-----
> From: John Caron [mailto:caron at unidata.ucar.edu]
> Sent: Tuesday, September 05, 2006 1:34 PM
> To: David Wojtowicz
> Cc: thredds at unidata.ucar.edu; 'support-thredds'
> Subject: Re: navigation information in GRIB2 datasets on THREDDS server
>
> The projection parameters under the Lambert_Conformal variable follow the CF
> conventions, and are in fact sufficient for georeferencing. These use the
> FGDC projection parameter name and meanings; we consider them more
> understandable than the GRIB values, but it sounds like you are more
> familiar with the GRIB values.
>
> David Wojtowicz wrote:
>
>> *Hi,*
>>
>>
>> *I'm having some trouble figuring out how to get all the
>> necessary projection information out of the GRIB2 datasets on
>> the motherlode THREDDS server.*
>>
>>
>> *I need to know the values of most of the items from the GRIB2
>> Grid Definition Section (section 3). For Lambert Conformal, the
>> following values should be available. (See Table 1 below)*
>>
>>
>> *However, I only see the navigation information on the THREDDS
>> server in the fragment from a ncdump of the dataset (in this
>> case NAM, 12KM) that is shown in table 2 below.*
>>
>>
>> *Specifically, I need:*
>>
>>
>> *Nx*,Ny (obvious from the grid)
>>
>>
>> *La1, Lo1 *
>>
>>
>> *ResCompFlag*
>>
>>
>> *Lov*
>>
>>
>> *Dx, Dy *
>>
>>
>> *Latin1, Latin2*
>>
>>
>> *Projection* Center Flag
>>
>>
>> *Scanning Mode*
>>
>>
>> *It's not clear how the values given in Table 2 (from the
>> THREDDS server) map to the values specified in the original
>> GRIB2 file as they have different names, and there doesn't
>> appear to be enough values to specify all of this. And even
>> where I can guess that the one value maps to the other, I'm not
>> sure if the units are compatible.*
>>
>>
>> *Any guidance would be appreciated. It is possible that the
>> values from the Grid Definition Section could simply be
>> represented as an integer array in THREDDS?*
>>
>>
>> *Thanks.*
>>
>>
>> *Table 1:*
>>
>>
>> *Grid Definition Template 3.30: Lambert conformal*
>>
>>Octet Number(s)
>>
>>
>>
>>Contents
>>
>>15
>>
>>
>>
>>Shape of the earth (see Code Table 3.2)
>><http://www.weather.gov/mdl/iwt/grib2/doc.htm#code3.2>
>>
>>16
>>
>>
>>
>>Scale factor of radius of spherical earth
>>
>>17-20
>>
>>
>>
>>Scaled value of radius of spherical earth
>>
>>21
>>
>>
>>
>>Scale factor of major axis of oblate spheroid earth
>>
>>22-25
>>
>>
>>
>>Scaled value of major axis of oblate spheroid earth
>>
>>26
>>
>>
>>
>>Scale factor of minor axis of oblate spheroid earth
>>
>>27-30
>>
>>
>>
>>Scaled value of minor axis of oblate spheroid earth
>>
>>31-34
>>
>>
>>
>>Nx - number of points along the X-axis
>>
>>35-38
>>
>>
>>
>>Ny - number of points along the Y-axis
>>
>>39-42
>>
>>
>>
>>La1 - latitude of first grid point
>>
>>43-46
>>
>>
>>
>>Lo1 - longitude of first grid point
>>
>>47
>>
>>
>>
>>Resolution and component flags (See Flag Table 3.3)
>><http://www.weather.gov/mdl/iwt/grib2/doc.htm#code3.3>
>>
>>48-51
>>
>>
>>
>>LaD - Latitude where Dx and Dy are specified
>>
>>52-55
>>
>>
>>
>>LoV - Longitude of meridian parallel to Y-axis along which latitude
>>increases as the Y- coordinate increases
>>
>>56-59
>>
>>
>>
>>Dx - X-direction grid length (see note (1))
>>
>>60-63
>>
>>
>>
>>Dy - Y-direction grid length (see note (1))
>>
>>64
>>
>>
>>
>>Projection centre flag (see Flag Table 3.5)
>><http://www.weather.gov/mdl/iwt/grib2/doc.htm#code3.5>
>>
>>65
>>
>>
>>
>>Scanning mode (see Flag Table 3.4)
>><http://www.weather.gov/mdl/iwt/grib2/doc.htm#code3.4>
>>
>>66-69
>>
>>
>>
>>Latin 1 - first latitude from the pole at which the secant cone cuts the
>>sphere
>>
>>70-73
>>
>>
>>
>>Latin 2 - second latitude from the pole at which the secant cone cuts
>>the sphere
>>
>>74-77
>>
>>
>>
>>Latitude of the southern pole of projection
>>
>>78-81
>>
>>
>>
>>Longitude of the southern pole of projection
>>
>>
>>
>>
>>
>>Table 2:
>>
>>
>>
>>char Lambert_Conformal ;
>>
>> Lambert_Conformal:grid_mapping_name =
>>"lambert_conformal_conic" ;
>>
>> Lambert_Conformal:standard_parallel = 25. ;
>>
>> Lambert_Conformal:longitude_of_central_meridian
>>= 265. ;
>>
>> Lambert_Conformal:latitude_of_projection_origin
>>= 25. ;
>>
>> Lambert_Conformal:GRIB_earth_shape = "Earth
>>spherical with radius of 6371229.0 m" ;
>>
>> Lambert_Conformal:GRIB_earth_shape_code = 6 ;
>>
>> Lambert_Conformal:DODS:strlen = 0 ;
>>
>> double y(y) ;
>>
>> y:units = "km" ;
>>
>> y:long_name = "y coordinate of projection" ;
>>
>> y:standard_name = "projection_y_coordinate" ;
>>
>> y:grid_spacing = "12.191001 km" ;
>>
>> y:_CoordinateAxisType = "GeoY" ;
>>
>> double x(x) ;
>>
>> x:units = "km" ;
>>
>> x:long_name = "x coordinate of projection" ;
>>
>> x:standard_name = "projection_x_coordinate" ;
>>
>> x:grid_spacing = "12.191001 km" ;
>>
>> x:_CoordinateAxisType = "GeoX" ;
>>
>>
>>
>>
>>
>>
>>
>>-----
>>David Wojtowicz, Sr. Research Programmer, Sysadmin
>>Dept of Atmospheric Sciences / Computer Services
>>University of Illinois at Urbana-Champaign
>>davidw at uiuc.edu <mailto:davidw at uiuc.edu> (217) 333-8390
>>
>>
>>
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