navigation information in GRIB2 datasets on THREDDS server

[John Caron]

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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