Guide to WMO Table Driven Code Forms

Code Table 5.4: Group Splitting Method
Code figure Meaning
0 Row by row splitting
1 General group splitting
2-191 Reserved
192-254 Reserved for local use
255 Missing
Row by row splitting is where each row of grid points is chosen to be a group. All group splitting
strategies fall into the General group splitting category.
Point 2: In addition to specifying a bit map in the Bit Map Section, missing data can be managed
explicitly in the Data Representation and Data Sections. This alternative permits added flexibility
in the description of missing data by distinguishing between a primary missing value and a
secondary missing value via reference to Code Table 5.5:
Code Table 5.5 Missing Value Management for Complex Packing
Code figure Meaning
0 No explicit missing values included within data values
1 Primary missing values included within data values
2 Primary and secondary missing values included within data values
3-191 Reserved
192-254 Reserved for local use
255 Missing
This concept may be useful, for example, to distinguish a missing value due to a land/sea mask
from a missing value for some other reason. If this approach is used, the appropriate code figure
from Code Table 5.5 is entered into octet 23 of the Data Representation Section, and the primary
and secondary missing value substitutes into octets 24-27 and 28-31, respectively. One way to
implement the use of a primary and secondary missing value would be to define the global
maximum field value +2 as the primary missing value, the global maximum field value +1 as the
secondary missing value, and make appropriate adjustments.
Points 5 and 6: Once the groups have been determined as noted under Point 1) above, the
smallest value in every group – the group reference value - are determined. The largest of the
group reference values is then found and the number of bits required to represent it is determined.
This number of bits is stored in octet 20 of the Data Representation Section. The group reference
values themselves - the X1 values in the GRIB2 compression formula - are stored in octets 6 – xx
of the Data Section.
Point 7 and 8: For each group, the group reference value calculated to provide the information
needed to satisfy Points 5) and 6) is subtracted from the Scaled Integers calculated under Step 5
of the Simple Packing method. These are the scaled values with the reference value removed -
the X2 values in the GRIB2 compression formula. They are stored in octets [zz + 1] – nn of the
Data Section using the appropriate group width calculated under Point 3 below.
Points 3 and 4: Group width is defined as the number of bits used for every second-order value in
a group. For each group, this is the number of bits required to represent the largest of the X2
values in that group. Group widths are calculated for all the groups in a field of grid point values.
The smallest of these group widths is stored in octet 36 of the Data Representation Section. This
minimum group width is then subtracted from every group width leaving an array of group width
increments. The number of bits required to represent the largest of these group width
(increments) is stored in octet 37 of the Data Representation Section. The group width
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