A user can define a GRIB product for dissemination which contains only data at selected points of a latitude-longitude grid or a Gaussian grid by defining a bitmap.

To support this extension, the following addition has been made to the command language for the definition of dissemination requirements:

BITMAP = "filename"

where

"filename" is the name of a separate file containing a valid bitmap specification.

For dissemination purposes, a bitmap specification takes the form of a set of command-like definitions containing the following:

SPEC a stand-alone command indicating that what follows is a bitmap specification

SIZE = yy:xx where yy indicates the number of rows,

xx indicates the number of points per row.

VALUES = OFF (default) indicates whether subsequent changes to the

ON bitmap are with respect to a bitmap preset to

OFF (i.e. 0 bits), or to ON (i.e. 1 bits)

POINTS = jj:kk where jj indicates row number, and

kk indicates which point(s) within a row take opposite value

to the preset value (VALUES).

kk can take the general form of a list of values: k1/k2/k3/../kn

where the format of a value is:

kk - depicting a single point

k1-k2 - points from k1 to k2 inclusive

0 - no points.

END a stand-alone directive terminating the specification.

The following example illustrates the use of the directives above. First the dissemination requirement:

DIS,COUNTRY=EC,TYPE=FC,LEVT=PL,LEVE=500,STEP=240,FORM=GRIB,

PARA=GH,REPR=LL,GRID=0.5/0.5,AREA=4.5/0.0/0.0/4.5,

BITMAP="ECM_001.BITMAP"

The bitmap specification is contained in a separate file, ECM_ 001.BITMAP, in the same directory as the file containing the dissemination requirements. It contains the following:

SPEC,SIZE=10:10,VALUES=OFF,

POINTS=01:03-08/10,

03:02-04/07-09,

04:02/03/08/09,

06:0,

07:01-03/08-10,

09:01-10

END

And gives the following bitmap:

0 0 1 1 1 1 1 1 0 1

0 1 1 1 0 0 1 1 1 0

0 1 1 0 0 0 0 1 1 0

0 0 0 0 0 0 0 0 0 0

1 1 1 0 0 0 0 1 1 1

1 1 1 1 1 1 1 1 1 1

Notes:

1. Within POINTS, values for rows remain defined until new values are given. Thus, in the example above:

- row 2 is the same as row 1;

- row 5 is the same as row 4;

- row 8 is the same as row 7;

- row 10 is the same as row 9.

2. The definition of row 6 simply indicates that no points are to be switched from the preset value defined in VALUES.

3. If VALUES=ON had been coded, the values in the bitmap would all be reversed, thus:

1 1 0 0 0 0 0 0 1 0

1 0 0 0 1 1 0 0 0 1

1 0 0 1 1 1 1 0 0 1

1 1 1 1 1 1 1 1 1 1

0 0 0 1 1 1 1 0 0 0

0 0 0 0 0 0 0 0 0 0

4. Member States may define a number of bitmaps in this way; it is suggested that, at least initially, this number be kept below about 20.

5. The same bitmap may be used for different areas, provided the areas conform with one another (i.e. have the same number of rows and columns).

18-01-2011