## ON388 - SECTION 1
THE PRODUCT DEFINITION SECTION (PDS)

Revised 7/10/03
Red text depicts changes made since 5/10/02

The PDS contains indicators for the Parameter table Version, the originating center, the numerical model (or "generating process") that created the data, the geographical area covered by the data, the parameter itself, the values for the appropriate vertical level or layer where the data reside, the decimal scale factor, and date/time information. The PDS is normally 28 octets long but it may be longer if an originating center chooses to make it so. Users of GRIB messages are strongly urged to use the length-of-section portion of the PDS to determine where the next section begins. Never assume a fixed octet length in this, or any other, section.

Octet no. | PDS Content
--- | ---
1-3 | Length in octets of the Product Definition Section
4 | Parameter Table Version number, currently 3 for international exchange. (Parameter table version numbers 128-254 are reserved for local use.)
5 | Identification of center (See Table 0)
6 | Generating process ID number (allocated by the originating center; See Table A)
7 | Grid Identification (geographical location and area, defined by the originating center; See Table B)
8 | Flag specifying the presence or absence of a GDS or a BMS (See Table 1)
9 | Indicator of parameter and units (See Table 2)
10 | Indicator of type of level or layer (See Tables 3 & 3a)
11-12 | Height, pressure, etc. of the level or layer (See Table 3)
13 | Year of century | Initial (or Reference) time of forecast - UTC or Start of time period for averaging or accumulation of analyses
14 | Month of year
15 | Day of month
16 | Hour of day
17 | Minute of hour
18 | Forecast time unit (see Table 4)
19 | P1 - Period of time (Number of time units) (0 for analysis or initialized analysis.) Units of time given by content of octet 18
20 | P2 - Period of time (Number of time units) or Time interval between successive analyses, successive initialized analyses, or forecasts, undergoing averaging or accumulation. Units given by octet 18.
21 | Time range indicator (See Table 5)
22-23 | Number included in average, when octet 21 (Table 5) indicates an average or accumulation; otherwise set to zero.
24 | Number Missing from averages or accumulations.
25 | Century of Initial (Reference) time (=20 until Jan. 1, 2001)
26 | Identification of sub-center (allocated by the originating center; See Table C)
27-28 | The decimal scale factor D A negative value is indicated by setting the high order bit (bit No. 1) in octet 27 to 1 (on).
29-40 | Reserved (need not be present)
41-... | Reserved for originating center use. (See Appendix C for information on NCEP's use of these octets for ensemble modeling.)

Note (1): Octet 8 may indicate the presence of the Grid Description Section (GDS) even though octet 7 specifies a predefined grid. In this case the GDS must describe that grid - this device serves as a mechanism for transmitting new "predefined" grids to users prior to their formal publication in this or the official WMO documentation. It is, however, the desired practice to always include the GDS in GRIB bulletins.
Note (2): The use of octet 26 to indicate a "sub-center" is now an officially sanctioned WMO practice. The use arises out of a recent change in the Manual in which the "originating center" for both GRIB and BUFR (FM 94) reference a single common table (WMO No. 306, Part C, Table C-1). The WMO will coordinate the assignment of the originating center numbers for national and international centers for both GRIB and BUFR, while each national center will then be free to assign sub-center numbers at will to be placed in the octet 26 of the GRIB PDS (or Octet 5 of BUFR Section 1). A zero value in octet 26 will serve as the default indicating that there is no sub-center associated with a particular center. Table 0, in this document, shows, in Part 1, the WMO recognized originating centers as would be found in octet 5, and, additionally, in Part 2, sub-center numbers allocated by NCEP.
Note (3): The NCEP Central Operations' (NCO) entries in the local use sections of Tables 2 and 6, as well as all NCO-defined tables, are specified in this Office Note.
Note (4): The NCEP currently uses three values for the parameter table version number (PDS octet 4), version number 2 and the new version numbers 129 and 130. Both of these tables are specified in this Office Note.

Office Note 388 - GRIB