Section 1 - Product definition section

      Octet
      No.
      Contents
      1-3Length of section
      4GRIB tables Version No. (currently 3 for international exchange) -- Version numbers 128-254 are reserved for local use
      5Identification of originating/generating center (see Code table 0)
      6Generating process identification number (allocated by originating center)
      7Grid definition (Number of grid used - from catalog defined by originating center) (see Notes 2, 3)
      8Flag (see Code table 1)
      9Indicator of parameter (see Code table 2)
      10Indicator of type of level (see Code table 3)
      11-12Height, pressure, etc. of levels (see Code table 3)
      13Year of century (see Note 6) Octets 13-17 give the Reference time of data - date and time of start of averaging or accumulation period
      14Month
      15Day
      16Hour
      17Minute
      18Indicator of unit of time range (see Code table 4)
      19P1 - Period of time (number of time units) (0 for analyses or initialized analyses). Units of time given by octet 18
      20P2 - Period of time (number of time units); or time interval between successive analyses, initialized analyses or forecasts, undergoing averaging or accumulation. Units of time given by octet 18
      21Time range indicator (see Code table 5)
      22-23Number included in average, when octet 21 (Code table 5) indicates an average or accumulation; otherwise set to zero
      24Number missing from averages or accumulations
      25Century of reference time of data (see Note 6)
      26Sub-center identification (see Code Table 0)
      27-28Units decimal scale factor (D)
      29-40Reserved: need not be present
      41-nnReserved for originating center use



      NOTES:

      1. Inclusion of Section 2 - Grid description section (GDS) is the preferred method of defining a grid.

      2. Where octet 7 defines a cataloged grid, that grid should also be defined in Section 2, provided the flag in octet 8 indicates inclusion of Section 2.

      3. Octet 7 must be set to 255 to indicate a non-cataloged grid, in which case the grid will be defined in Section 2.

      4. A negative value of D shall be indicated by setting the high-order bit (bit 1) in the left-hand octet to 1 (on).

      5. If a Grid description section is not included, then any u- or v-components of vector quantities in the message are to be resolved relative to the specified grid in the direction of increasing x and y (or i and j) coordinates, respectively.
        If a Grid description section is included in the message, which is the preferred option, then octet 17 of the GDS and Code table 7 will contain component resolution information.

      6. To specify year 2000, octet 13 of the section (Year of the century) shall contain a value equal to 100 and octet 25 of the section (Century of reference time data) shall contain a value equal to 20. To specify year 2001, octet 13 of the section shall contain a value equal to 1 and octet 25 of the section shall contain a value equal to 21 (by International Convention, the date of 1 January 2000 is the first day of the hundredth year of the twentieth century and the date of 1 January 2001 is the first day of the first year of the twenty-first century); it is to be noted also that year 2000 is a leap year and the 29 February 2000 exists.



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