The purpose of the (optional) GDS is to provide a grid description for grids not defined by number in Table B. 
Octet no.  GDS Content 


Length in octets of the Grid Description Section 

NV, the number of vertical coordinate parameters 

PV, the location (octet number) of the list of vertical
coordinate parameters, if present or PL, the location (octet number) of the list of numbers of points in each row (when no vertical parameters are present), if present or 255 (all bits set to 1) if neither are present 

Data representation type (see Table 6) 

Grid description, according to data representation type, except Lambert, Mercator, Space View, or Curvilinear Orthogonal grids (see Table D). 



Grid description for Lambert or Mercator grid (see Table D) 



Grid description for Space View perspective grid (see Table D) 

List of vertical coordinate parameters (length = NV x 4 octets); if present, then PL = 4 x NV + PV 

List of numbers of points in each row, used for quasiregular grids (length = NROWS x 2 octets, where NROWS is the total number of rows defined within the grid description) 
Note: NV and PV relate to features of GRIB not, at present, in use in the National Weather Service. See the WMO Manual on Codes for the descriptions of those features. PL is used for "quasiregular" or "thinned" grids; e.g., a lat/lon grid where the number of points in each row is reduced as one moves poleward from the equator. The reduction usually follows some mathematical formula involving the cosine of the latitude, to generate an (approximately) equally spaced grid array. The association of the numbers in octet PL (and following) with the particular row follows the scanning mode specification in Table 8. 
NOTES ON SPECTRAL TRUNCATION:
Using the associated Legendre Polynomials of the First Kind, P_{n}^{m}, as typical expansion functions, any variable x(,), which is a function of longitude, , and sin(latitude), , can be represented by In the summations, M is the maximum zonal wave number that is to be included, and K & J together define the maximum meridional total wave number N(m), which, it should be noted, is a function of m. A sketch shows the relationships: In this figure, the ordinate, n, is the zonal wave number, the abscissa, m, is the total meridional wave number, the vertical line at m = M is the zonal truncation, and the diagonal passing through (0,0) is the line n = m. The Legendre Polynomials are defined only on or above this line, that is for n > m. On the naxis, the horizontal line at n = K indicates the upper limit to n values, and the diagonal that intersects the naxis at n = J indicates the upper limit of the area in which the Polynomials are defined. The shaded irregular pentagon defined by the naxis, the diagonal from n = J, the horizontal n = K, the vertical m = M, and the other diagonal n = m surrounds the region of the (n x m) plane containing the Legendre Polynomials used in the expansion. This general pentagonal truncation reduces to some familiar common truncations as special cases:
In all of the above m can take on negative values to represent the imaginary part of the spectral coefficients. 