navigation information in GRIB2 datasets on THREDDS server

[David Wojtowicz]

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
<http://www.weather.gov/mdl/iwt/grib2/doc.htm#code3.2>  3.2)


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
<http://www.weather.gov/mdl/iwt/grib2/doc.htm#code3.3>  3.3)


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
<http://www.weather.gov/mdl/iwt/grib2/doc.htm#code3.5>  3.5)


65

Scanning mode (see Flag Table
<http://www.weather.gov/mdl/iwt/grib2/doc.htm#code3.4>  3.4)


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  (217) 333-8390