Guide to WMO Table Driven Code Forms

GRIB1 Sections GRIB2 Sections
Indicator Section Indicator Section
No counterpart Local Use Section
¦-------> Identification Section
Product Definition Section Product Definition Section
¦-------> Data Representation Section
Grid Description Section Grid Definition Section
Bit-Map Section Bit-Map Section
Binary Data Section Data Section
End Section End Section
The subdivision of the GRIB1 Product Definition Section into the GRIB2 Identification, Product
Definition, and Data Representation Sections is one of the fundamental differences between
GRIB1 and GRIB2. This, combined with the option for iterating the Sections (described next) and
expansion of the concept of templates (described in Item 1.2.2), are what provide GRIB2 with its
substantially enhanced flexibility over what is possible in GRIB1.
1.2.1.2 Iterating the Sections
GRIB1 only permits one field to be represented by a single GRIB1 message. However, GRIB2
achieves additional flexibility by allowing more than one field to be represented in a single GRIB2
message. This is allowed by permitting specific groups of sections to be repeated within a GRIB2
message. Consider the sections of a GRIB2 message referred to above:
Section 0: Indicator Section
Section 1: Identification Section
Section 2: Local Use Section (optional) |
Section 3: Grid Definition Section | |
Section 4: Product Definition Section | | |
Section 5: Data Representation Section | repeated |repeated |repeated
Section 6: Bit-Map Section | | |
Section 7: Data Section | | |
Section 8: End Section
The GRIB2 regulations state that (A) Sequences of GRIB2 sections 2 to 7, 3 to 7, or 4 to 7 may be
repeated within a single GRIB2 message, (B) All sections within such repeated sequences must
be present and shall appear in the numerical order noted above, and (C) Unrepeated sections
remain in effect until redefined. This is denoted schematically in the above table by the three
groups of vertical bars.
Consider, for example, 500 hPa height field forecasts on a Northern Hemisphere polar
stereographic grid produced by a particular numerical model at forecast hours 12, 24, 36, and 48.
These four fields could be represented by a single GRIB2 message by repeating the sequence of
Sections 4 to 7 four times, making the appropriate forecast time changes in the Product Definition
Section in each iteration of the sequence. This GRIB2 message would appear schematically as
follows:
4