# Section 4: PRODUCT DEFINITION SECTION

Octets | Key | Type | Content |

1-4 | section4Length | unsigned | Length of section in octets (nn) |

5 | numberOfSection | unsigned | Number of section (4) |

6-7 | NV | unsigned | Number of coordinate values after template or number of information according to 3D vertical coordinate GRIB2 message |

8-9 | productDefinitionTemplateNumber | codetable | Product Definition Template Number (see Code Table 4.0) |

10-xx | Product Definition Template (see Template 4.X, where X is the Product Definition Template Number given in octets 8-9) | ||

[xx+1]-nn | pv | ieeefloat | Optional list of coordinate values or vertical grid information |

( 1) Coordinates values are intended to document the vertical discretisation associated with model data on hybrid coordinate vertical levels. A number of zero in octets 6-7 indicates that no such values are present. Otherwise the number corresponds to the whole set of values.

( 2) Hybrid systems, in the context, employ a means of representing vertical coordinates in terms of a mathematical combination of pressure and sigma coordinates. When used in conjunction with a surface pressure field and an appropriate mathematical expression, the vertical coordinate parameters may be used to interpret the hybrid vertical coordinate.

( 3) Hybrid coordinate values, if present, should be encoded in IEEE 32-bit floating point format. They are intended to be encoded as pairs.

( 4) Two distinct pressure-based hybrid coordinate formulations can be expressed in GRIB Edition 2. If the hybrid coordinate being used is based on pressure, then level type 105 (Code table 4.5) shall be used to specify the vertical level type. If the formulation is based on the natural logarithm of pressure then level type 113 (Code table 4.5) shall be used. In both cases Notes 1 to 3 (above) apply fully.

( 5) In the case of a generalized vertical height coordinate (fixed surface type 150), no pairs of coordinate values follow after the template, but six sets of additional information (each 4 octets long and encoded in IEEE 32-bit floating point format) follow, starting with the number of vertical levels and the identification number of the used vertical system in the additional GRIB2 message with the 3D vertical system. This identification number together with an UUID (Universally Unique IDentifier) in four parts allows a unique identification of the grid.

- [xx+1] - [xx+4] Number of vertical levels
- [xx+5] - [xx+8] Identification number of 3D vertical grid GRIB2 message
- [xx+9] - [xx+12] UUID part 1 of 4
- [xx+13] - [xx+16] UUID part 2 of 4
- [xx+17] - [xx+20] UUID part 3 of 4
- [xx+21] - [xx+24] UUID part 4 of 4