GRIB API Manual

0          A bitmap is included in the GRIB message.
           The data array PSEC4 contains a value which serves as a
           missing data indicator for use at missing data points.

Non-zero   The number of a predefined bitmap.
           The bitmap is not included in the message. The binary
           data array contains only valid data values; missing
           points are suppressed.

bitmapPresent

The value used at missing data points in an INTEGER data field. This value must be supplied for both encoding and decoding.

missingValue

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