Section 1: Product definition section

Octets Key Type Content
1-3 section1Length unsigned Length of section
4 table2Version unsigned GRIB tables Version No. (currently 3 for international exchange) Version numbers 128-254 are reserved for local use
5 centre codetable Identification of originating/generating centre (see Code table 0 = Common Code table C1 in Part C/c.)
6 generatingProcessIdentifier unsigned Generating process identification number (allocated by originating centre)
7 gridDefinition unsigned Grid definition (Number of grid used from catalogue defined by originating centre)
8 section1Flags codeflag Flag (see Regulation 92.3.2 and Code table 1)
9 indicatorOfParameter codetable Indicator of parameter (see Code table 2)
10 indicatorOfTypeOfLevel codetable Indicator of type of level (see Code table 3)
11-12 Height, pressure, etc. of levels (see Code table 3)
13 yearOfCentury unsigned Year of century
14 month unsigned Month      Reference time of data date and time of
15 day unsigned Day          start of averaging or accumulation period
16 hour unsigned Hour
17 minute unsigned Minute
18 unitOfTimeRange codetable Indicator of unit of time range (see Code table 4)
19 P1 unsigned P1 Period of time (number of time units) (0 for analyses or initialized analyses). Units of time given by octet 18
20 P2 unsigned P2 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
21 timeRangeIndicator codetable Time range indicator (see Code table 5)
22-23 numberIncludedInAverage unsigned Number included in average, when octet 21 (Code table 5) indicates an average or accumulation; otherwise set to zero
24 numberMissingFromAveragesOrAccumulations unsigned Number missing from averages or accumulations
25 centuryOfReferenceTimeOfData unsigned Century of reference time of data
26 subCentre codetable Sub-centre identification (see common Code table C1 in Part C/c., Note (3))
27-28 decimalScaleFactor signed Units decimal scale factor (D)
29-40 Reserved: need not be present
41-nn Reserved for originating centre use

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

( 2) Where octet 7 defines a catalogued 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-catalogued 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 that 29 February 2000 exists.