GRIB section 3, the bitmap section

An INTEGER array of at least 2 elements.


ElementContentsecCodes keys
1
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
2


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

Contents of KSEC3