Data Template 7.2 - Grid Point Data - Complex Packing

Contents

• 6-xx: NG group reference values (X1 in the decoding formula), each of which is encoded using the number of bits specified in octet 20 of data representation template 5.0. Bits set to zero shall be appended as necessary to ensure this sequence of numbers ends on an octet boundary
• [xx+1]-yy: NG group widths, each of which is encoded using the number of bits specified in octet 37 of data representation template 5.2. Bits set to zero shall be appended as necessary to ensure this sequence of numbers ends on an octet boundary
• [yy+1]-zz: NG scaled group lengths, each of which is encoded using the number of bits specified in octet 47 of data representation template 5.2. Bits set to zero shall be appended as necessary to ensure this sequence of numbers ends on an octet boundary (see Note 5)
• [zz+1]-nn: Packed vaules (X2 in the decoding formula), where each value is a deviation from its respective group reference value

Notes

• (1) Group descriptors mentioned above may not be physically present; if associated field width is 0.
• (2) Group lengths have no meaning for row by row packing; for consistency, associated field width should then be encoded as 0. So no specific test for row case is mandatory at decoding software level to handle endcoding/decoding of group descriptors.
• (3) Scaled group lengths, if present, are encoded for each group. But the true last group length (unscaled) should be taken from data representation template.
• (4) For groups with a constant value, associated field width is 0, and no incremental data are physically present.
• (5) The essence of the complex packing method is to subdivide a field of values into NG groups, where the values in each group have similar sizes. In this procedure, it is necessary to retain enough information to recover the group lengths upon decoding. The NG group lengths for any given field can be described by Ln = ref + Kn x len_inc, n = 1, NG, where ref is given by octets 38 - 41 and len_inc by octet 42. The NG values of K (the scaled group lengths) are stored in the data section, each with the number of bits specified by octet 47. Since the last group is a special case which may not be able to be specified by this relationship, the length of the last group is stored in octets 43-46.

Data Template 7.3 - Grid Point Data - Complex Packing and Spatial Differencing

Contents

• 6-ww: First value(s) of original (undifferenced) scale values, followed by the overall minimum of the differences. The number of values stored is 1 greater than the oerder of differentiation, and the field width is described at octet 49 of data representation template 5.3 (see Note 1)
• [ww+1]-xx: NG group difference values, (X1 in the decoding formula), each of which is encoded using the number of bits specified in octet 20 of data representation template 5.0. Bits set to zero shall be appended where necessary to ensure this sequence of numbers ends on an octet boundary
• [xx+1]-nn: Packed vaules (X2 in the decoding formula), where each value is a deviation from its respective group reference value

Notes

• (1) Referring to the notation in Note 1 of data representation template 5.3, at order 1, the values stored in octet 6-ww are g1 and gmin. At order 2, the values stored are h1, h2 and hmin.
• (2) Extra descriptors related to spatial differencing are added before the splitting descriptors, to refect the separation between the two approaches. It enables to share software parts between cases with and without spatial differencing.
• (3) The position of overall minimum after initial data values is a choice that enables less software management.
• (4) Overall minimum will be negative in most cases. First bit should indicate the sign:0 if positive, 1 if negative.