Guide to WMO Table Driven Code Forms

1.2.2 Code Templates
Although not referred to as such, the template concept is used in GRIB1 in the Grid Description
Section, where octets 6 – 32 are defined as follows:
Octet 6 Data representation type (see Table 6)
Octets 7 – 32 Grid definition (according to data representation type – octet 6
above)
Thus, the contents of octets 7 – 32 are not explicitly defined. Rather, the value in octet 6 refers to
an entry in GRIB1 Table 6, and that entry defines the contents of octets 7 – 32. In the
nomenclature of GRIB2, the contents of octet 6 would be referred to as the template number, and
the contents of octets 7 – 32 would be referred to as the template. In GRIB2, this powerful
concept has been retained in the Grid Definition Section, but expanded to product definition and
data representation as well. It has also been made more flexible by leaving variable the number of
octets used for the template. The separation of identification, product definition and data
representation into separate sections facilitates this application of the template concept in GRIB2.
Thus, there are three categories of GRIB2 templates – Grid Definition Templates, Product
Definition Templates, and Data Representation Templates. They are identified by the Grid
Definition Template Number, Product Definition Template Number, and Data Representation
Template Number, respectively. These template numbers are coded in the appropriate section.
Two octets are allocated for each template number, ensuring table space will not become
exhausted.
When a new product, new grid, or new type of data representation is needed, one has to simply
define the appropriate new template(s) and submit them to the ICT/DRC for approval (the approval
process is described in Section 1.3 of this Layer). With ample table space allowed, GRIB2 thus
possesses the flexibility to represent virtually any product using virtually any type of data
representation on virtually any type of grid projection both now and in the future.
The templates themselves are somewhat complicated. They are described in detail in Layer 2 of
this Guide.
1.2.3 Tables
Because of the extensive use of templates, there are two categories of tables in GRIB2 –
Template Tables and Code and Flag Tables. The tables in both categories are numerous and, at
times, complicated. In fact, the tables form by far the largest part of the documentation of GRIB2
in the Manual on Codes. Both types of tables are described briefly below and in detail in Layer 2
of this Guide.
1.2.3.1 Template Tables
All templates are described in the Template Tables. These Tables are categorized by the Section
number in which the templates are found. In the tables, the templates themselves are labelled
m.n, where m is the number of the section is which the template is found and n is the template
number coded in the appropriate section. The name of the template is given by a code table and
is also referred to by the template number n. The first quantity in each template is in the octet
specified in its respective section. Although the templates have variable lengths, the last octet of
the template can always be calculated from values coded in the section.
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