Grid definition - latitude/longitude grid (or equidistant cylindrical)



Octet No.	Contents
7-8	Ni - number of points along a parallel
9-10	Nj - number of points along a meridian
11-13	La1 - latitude of first grid point
14-16	Lo1 - longitude of first grid point
17	Resolution and component flags (see Code table 7)
18-20	La2 - latitude of last grid point
21-23	Lo2 - longitude of last grid point
24-25	Di - i direction increment
26-27	Dj - j direction of increment
28	Scanning mode (flags - see Code table 8)
29-32	Set to zero (reserved)
33-35	Latitude of the southern pole in millidegrees (integer)
	Latitude of pole of stretching in millidegrees (integer)
36-38	Longitude of the southern pole in millidegrees (integer)
	Longitude of pole stretching in millidegrees (integer)
39-42	Angle of rotation (represented in the same way as the reference value)
	Stretching factor (representation as for the reference value)
43-45	Latitude of pole of stretching in millidegrees (integer)
46-48	Longitude of pole of stretching in millidegrees (integer)
49-52	Stretching factor (representation as for the reference value)





NOTES:


1. Latitude, longitude and increments are in millidegrees.
   
   
2. Latitude values are limited to the range 0-90000; bit 1 is set to 1 to indicate south latitude.
   
   
3. Longitude values are limited to the range 0-360000; bit 1 is set to 1 to indicate west longitude.
   
   
4. The latitude and longitude of the last grid point and the first grid point should always be given for regular grids.
   
   
5. Where items are not given, the appropriate octet(s) should have all bits set to 1.
   
   
6. Three parameters define a general latitude/longitude coordinate system, formed by a general rotation of the sphere. One choice for these parameters is:
   1. The geographic latitude in degrees of the southern pole of the coordinate system, theta_p for example;
   2. The geographic longitude in degrees of the southern pole of the coordinate system, lamda_p for example;
   3. The angle of rotation in degrees about the new polar axis (measured clockwise when looking from the southern to the northern pole) of the coordinate system, assuming the new axis to have been obtained by first rotating the sphere through lamda_p degrees about the geographic polar axis, and then rotating through (90 + theta_p ) degrees so that the southern pole moved along the (previously rotated) Greenwhich meridian.
   
   
   
7. For rotated grids, the vertical coordinate parameters start at octet 43 instead of 33.
   
   
8. The stretching is defined by three parameters:
   1. The latitude in degrees (measured in the model coordinate system) of the "pole of stretching";
   2. The longitude in degrees (measured in the model coordinate system) of the "pole of stretching";
   3. The stretching factor C.
   The stretching is defined by representing data uniformly in a coordinate system with longitude lamda and latitude theta¹, where: theta¹ = sin^-1 ( [ (1 - C²) + (1 + C²) sin theta] / [ (1 + C²) + (1-C²) sin theta] ) and lamda and theta are longitude and latitude in a coordinate system in which the "pole of stretching" is the northern pole. C = 1 gives uniform resolution, while C > 1 gives enhanced resolution around the pole of stretching.
   
   
9. For stretched grids, the vertical coordinate parameters start at octet 43 instead of 33.
   
   
10. For stretched and rotated latitude/longitude grids, the vertical coordinate parameters start at octet 53.
    
    
11. The first and last grid points may not necessarily correspond to the first and last data points, respectively, if the bit-map section is used.
    
    
12. For data on a quasi-regular grid, in which all the rows or columns do not necessarily have the same number of grid points, either Ni (octets 7-8) or Nj (octets 9-10) and the corresponding Di (octets 24-25) or Dj (octets 26-27) shall be coded with all bits set to 1 (missing); the actual number of points along each parallel or meridian shall be coded.
    
    
13. A quasi-regular grid is only defined for appropriate grid scanning modes. Either rows or columns, but not both simultaneously, may have variable numbers of points. The first point in each row (column) shall be positioned at the meridian (parallel) indicated by octets 11-16. The grid points shall be evenly spaced in latitude (longitude).



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