## ON388 - SECTION 3
BIT MAP SECTION (BMS)

Revised 1/2/96

The purpose of the (optional) BMS is to provide either a bit map or a reference to a bit map pre-defined by the center. The bit map consists of contiguous bits with a bit-to-data-point correspondence as defined in the grid description. A bit set equal to 1 implies the presence of a datum for that grid point in the BDS; a value of zero implies the absence of such. This is useful in shrinking fields where fair portions of the field are not defined. An example would be global grids of sea surface temperature; the bit map would be used to suppress the "data" at grid points over land. One would not want to use the BMS if the data were undefined at only a small number of grid points as the overhead of adding the bit map array (one bit for each grid point) might add more bits to the overall message that were subtracted by the removal of a few data values.

Octet no. | BMS Content
--- | ---
1-3 | Length in octets of Bit Map Section
4 | Number of unused bits at end of Section 3.
5-6 |
Numeric: | 0 - | a bit map follows
| otherwise - | the numeric refers to a predefined bit map provided by the center
7-nnn | Bit map, zero filled to an even number of octets

Office Note 388 - GRIB