GRIB2 - DATA REPRESENTATION TEMPLATE 5.61

Grid point data - Simple packing with logarithm pre-processing

Created 01/19/2012

Preliminary note: This template is experimental, was not validated at the time of
publication and should be used only for bilateral previously agreed tests.


Octet No. Contents
12-15
Reference value (R) (IEEE 32-bit floating-point value)
16-17
Binary scale factor (E)
18-19
Decimal scale factor (D)
20 Number of bits used for each packed value
21-24 Pre-processing parameter (B) (IEEE 32-bit floating-point value)


Notes:

(1) The template is approriately designed for data sets with all non-negative values and a wide variability range (more then 5 orders of magnitude). It must not be used for data sets with negative values or smaller variability range.

(2) A logarithm pre-processing algorithm is used to fit the variability range into one or two order of magnitudes before using the simple packing algorithm. It requires a parameter (B) to assure that all values passed to the logarithm function are positive. Thus scaled values are Z=ln (Y+B), where Y are the original values, ln is the natural logarithm (or Napierian) function and B is chosen so that Y=B>0.

(3) Best pratice follows for choosing the B pre-processing parameter.
   (a) If the data set minimum value is positive, B can be safely put to zero.
   (b) If the data set minimum is zero, all values must be scaled to become greater than zero and B can be equal to the minimum positive value in the data set.

(4) Data shall be packed using Data template 7.


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