To illustrate how templates change / extend the content of a GRIB2 message, the following figure shows an example of the product definition templates 4.0 and 4.1.
Template 4.0: instantaneous deterministic product | Template 4.1: instantaneous probabilistic product | ||
10 | Parameter category (Code table 4.1) | 10 | Parameter category (Code table 4.1) |
|---|---|---|---|
11 | Parameter number (Code table 4.2) | 11 | Parameter number (Code table 4.2.0.0 for temperature) |
12 | Type of generating process (Code table 4.3) | 12 | Type of generating process (Code table 4.3) |
13 | Background generating process identifier | 13 | Background generating process identifier |
14 | Forecast generating process identifier | 14 | Forecast generating process identifier |
15-16 | Hours after reference time of data cut-off | 15-16 | Hours after reference time of data cut-off |
17 | Minutes after reference time of data cut-off | 17 | Minutes after reference time of data cut-off |
18 | Indicator of unit of time range ( Code table 4.4) | 18 | Indicator of unit of time range (Code table 4.4) |
19-22 | Forecast time in units defined by octet 18 | 19-22 | Forecast time in units defined by octet 18 |
23 | Type of first fixed surface (Code table 4.5) | 23 | Type of first fixed surface (Code table 4.5) |
24 | Scale factor of first fixed surface | 24 | Scale factor of first fixed surface |
25-28 | Scaled value of first fixed surface | 25-28 | Scaled value of first fixed surface |
29 | Type of second fixed surface (Code table 4.5) | 29 | Type of second fixed surface (Code table 4.5) |
30 | Scale factor of second fixed surface | 30 | Scale factor of second fixed surface |
31-34 | Scaled value of second fixed surface | 31-34 | Scaled value of second fixed surface |
35 | Type of ensemble forecast (Code table 4.6) | ||
36 | Perturbation number | ||
37 | Number of forecasts in ensemble | ||
The number in the first column means that at this/(these) specific byte(s) in the section 4, the coded integer value(s) either refer to a specific WMO table, which is specified in the brackets, or contain a value for a specific key like the Forecast time.
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The section length is encoded at the beginning of each section so that the decoder knows how long each section is. For template 4.0, the section 4 length is 34 and for template 4.1 37 bytes long. Which template is used, is encoded in the section itself.
The key "key parameterNumber" doesn't have a Code Table to itself, meaning the Code Table 4.2 does not exist. Instead we have is divided into subtables 4.2.[discipline].[parameterCategory], for example 4.2.0.0 for temperature.
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