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.