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When data are packed in GRIB using the simple packing method, the values are packed by storing differences from the reference value (the minimum of all the values) to a specified fixed number of bits referred to as the "bits per value" (ecCodes key: bitsPerValue).  Thus the packing introduces a discretisation of the values with an associated packing error (ecCodes key:  packingError) such that:

\mathrm{\mathbf{packed\ value}} - \mathrm{\mathbf{packing\ error}} < \mathrm{\mathbf{true\ value}} =\leq \mathrm{\mathbf{packed\ value}} \pm+ \mathrm{\mathbf{packing\ error}}

The packing error depends on the range of vafieldslues field values and the number of bits per value used to pack the values.

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• All accumulations start with a zero value at step=0
• The values at all points are packed exactly

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• The model values increase at all points except the north west
• Not all values can be represented exactly using 8 bits per value
• In particular, the value of 0.031250 at the southern point is exactly the packing error and its packed value becomes 0.062500
• In this case, the packed values are all multiples of 2*packing error=0.0625 so values of 4.55, 5.9 and 2.2 are not stored precisely

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• Only the model value at the north east point increases

• The increase in the range of values increases the packing error by a factor of 2

• Now all packed values are multiples of 2*packing error=0.125

• The value at the central point of 4.55 is now packed as 4.5 compared to a packed value of 4.5625 at the previous step - a negative accumulation of -0.0625
• The value at the south west point of 2.2 is now packed as 2.25 compared to a packed value of 2.1857 at the previous step - a positive accumulation of +0.0625

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• Again only the model value at the north east point increases further from 20.0 to 40.0
• The increase in the range of values increases the packing error.
• Now all packed values are multiples of 2*packing error=0.25
• The model value at the eastern point is unchanged at 5.9 but now its packed value is 6.0 giving an apparent positive accumulation  of +0.125

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