Re: navigation information in GRIB2 datasets on THREDDS server

• To: David Wojtowicz <davidw@xxxxxxxx>
• Subject: Re: navigation information in GRIB2 datasets on THREDDS server
• From: John Caron <caron@xxxxxxxxxxxxxxxx>
• Date: Tue, 05 Sep 2006 12:34:09 -0600

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@xxxxxxxx <mailto:davidw@xxxxxxxx>  (217) 333-8390

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