This writeup describes the format of tapes containing grid analysis data from the U.S. Navy Fleet Numerical Oceanography Center (formerly Fleet Numerical Weather Central). These analyses include the following types of grids:

Grid Name	Dimensions	Number of Points	Grid Coverage
Northern Hemisphere	63 x 63	3969	north pole to about 0.2N
Southern Hemisphere	63 x 63	3969	south pole to about 0.2S
Northern Hemisphere	125 x 125	15625	double resolution of 63x63
Southern Hemisphere	125 x 125	15625	double resolution of 63x63
Global Spherical	144 x 73	10512	entire globe
Global Band	144 x 49	7056	tropical band 40.9S to 59.7N

They are packed according to the format given below. The routine GBYTES has been written for several computers, is documented in NCAR TN/IA-93, and makes the unpacking of binary data a simple task.

These grids have not been converted to a standard set of units, partly because of lack of time, and partly because of the loss of precision during such conversions.

1. Load point on the tape.
   
2. One tape block (physical record) for each data grid (logical record). Each block contains the identification along with the data. There are no separate tape label records. The blocks are written in binary mode (odd parity).
   
3. An end of file (EOF) follows the last data record.

1. Words 1 and 2 are the grid label:

   FIELD	form	year	mon	day	hour	pres	var	fore	misca	phys rec	source	stat	bias	scale	miscb
   BITS PRIOR TO FIELD	0	6	13	17	22	27	37	46	55	65	67	73	78	94	110
   FIELD WIDTH (BITS)	6	7	4	5	5	10	9	9	10	2	6	5	16	16	10

   where:

   Field	Description
   form	format number:   3 - Northern Hemisphere 63 x 63 4 - Southern Hemisphere 63 x 63 10 - Global Band 144 x 49 11 - Global Spherical 144 x 73 13 - Northern Hemisphere 125 x 125 14 - Southern Hemisphere 125 x 125
   year	year without century, e.g. 65 = 1965
   mon	month, 1 to 12 for January to December
   day	day of month, 1 to 31. On a monthly mean grid, this value is 0 - see miscb.
   hour	Greenwich mean time of analysis, 00Z, 06Z, 12Z or 18Z
   pres	pressure level in millibars. Levels with higher pressure values generally appear first on the tape. Sea level pressure uses 1013. Surface oceanographical is 1013. Surface meteorological is 1001. Upper air levels are: 1000, 925, 850, 700, 500, 400, 300, 250, 200, 100, 50, 30 & 10. See Table 1.
   var	variable code such as 1 for heights, 10 for temperatures See Tables 1 and 2 for availability, codes, and units.
   fore	0 for an analysis, non-zero for forecast
   misca	unused (intended for second pressure of thickness fields)
   phys rec	unused (artifact from old three-record format, the presence of which requires modification of RGRID type inventory)
   source	source of the analysis, 3 = U.S. Navy F.N.O.C.
   stat	on daily grids this is the grid status or quality: 0 - OK, no known problems non-zero - there are problems, but the user may find some of the grid to be useful on monthly grids it equals (miscb / 4.) + .5
   bias	the bias value used in packing the data so that all packed values are positive. 32768 for most grids.
   scale	scale value used in packing grid values, packed as bias + scale, so that -6 for SST is packed as 32762
   miscb	unused on daily grids. On monthly mean grids this value is the number of days used to make the mean grid.

   
   

2. Word 3 is a CDC 7600 floating point value for the Base Value. See paragraph 4 for its incorporation in the unpacking. This value is usually zero. For height grids, it contains the ICAO standard height for the level involved.
   
   
3. Words 4 through the word shown below hold the packed data points, as a continous string of 16-bit bytes. The last 60-bit word is zero padded. The last 60-bit word used, by grid size:

   Grid	Number of Points	In Words 4 Through	Unused Bits (0 Padding)
   63 x 63	3969	1062	36
   125 x 125	15625	4170	20
   144 x 73	10512	2807	48
   144 x 49	7056	1885	24

   
   
4. Following the last 60-bit word containing the packed data points, is a 60-bit checksum obtained by adding all the preceding 60-bit words. This addition is an add and carry logical addition (as performed by a CDC 7600)
5. Following the checksum is up to 16 60-bit words containing parts of the original Navy identification information plus some extraneous bits found at the end of the original data records. At this time, these are of no interest to the user, except as a consideration for defining a maximum buffer (or tape block) size.

The data can be unpacked with the folowing equation, which is used in the RUNAVY routines:

Figures 1 through 6 show the general layout of the grid data points. The indexing for the grids is as follows:

1. 63 x 63 or 125 x 125. Origin, GRID(1,1) at lower left. For GRID(I,J): J=1, I scans from left to right, then J steps up 1, so that:

   63 X 63		125 X 125
   Packed Point	Grid I,J	Packed Point	Grid I,J
   1	1,1	1	1,1
   2	2,1	2	2,1
   3	3,1	3	3,1
   ...	...	...	...
   63	63,1	125	125,1
   64	1,2	126	1,2
   ...	...	...	...
   1985	32,32	7813	63,63	(Pole)
   ...	...	...	...
   3967	61,63	15623	123,125
   3968	62,63	15624	124,125
   3969	63,63	15625	125,125

   The 63 x 63 grid is an extension of the NMC 47 x 51 octagonal grid. The pole point on the Navy grid, I,J = 32,32 corresponds to I,J = 24,26 on the NMC grid.
   
   
2. 144 x 73 or 144 x 49. Origin, GRID(1,1) at upper left. For GRID(I,J): J=1, I scans from top to bottom, then J steps right 1, so that:

   144 X 73			144 X 49
   Packed Point	Grid I,J	Lat/Lon	Packed Point	Grid I,J	Lat/Lon
   1	1,1	90.0 N, 60.0 E	1	1,1	59.7 N, 60.0 E
   2	2,1	87.5 N, 60.0 E	2	2,1	58.6 N, 60.0 E
   3	3,1	85.0 N, 60.0 E	3	3,1	57.4 N, 60.0 E
   ...	...		...	...
   37	37,1	0.0 N, 60.0 E		31,1	0.0 N, 60.0 E
   ...	...		...	...
   72	72,1	87.5 S, 60.0 E	48	48,1	39.0 S, 60.0 E
   73	73,1	90.0 S, 60.0 E	49	49,1	40.9 S, 60.0 E
   74	1,2	90.0 N, 62.5 E	50	1,2	59.7 N, 62.5 E
   ...	...		...	...
   8759	72,120	87.5 S, 357.5 E	5879	48,120	39.0 S,357.5 E
   8760	73,120	90.0 S, 357.5 E	5880	49,120	40.9 S,357.5 E
   8761	1,121	90.0 N, 0.0 E	5881	1,121	59.7 N, 0.0 E
   ...	...		...	...
   10510	71,144	85.0 S, 57.5 E	...	...
   10511	72,144	87,5 S, 57.5 E	7055	48,144	39.0 S, 57.5 E
   10512	73,144	90.0 S, 57.5 E	7056	49,144	40.9.S, 57.5 E

1. Showing the grid label and the first data point (DP#1)

   Data	Packed Binary Value	Octal	Base 10	Meaning
   form	00011	3	3	63 x 63 grid
   year	1000001	101	65	1965
   mon	0110	6	6	June
   day	11101	35	29	29th
   hour	01100	14	12	12Z
   pres	0000001010	12	10	1023 - 173 = 850mbs
   var	000111010	71	57	sea surface temperature
   fore	000000000	0	0	an analysis
   misca	0000000000			not used
   phys rec	00			no longer used
   source	000011	3	3	U.S. Navy
   stat	00000	0	0	no known problems with the grid
   bias	1000000000000000	100000	32768	see unpacking formula
   scale	0111111111111010	77772	32762	Scale = 32762 - 32768 = -6
   miscb	0000000000			not used on a daily
   base	60 bits of zero	0	0	see unpacking formula
   DP#1	1000011001000111	103107	34375	= 0 + (34375 - 32768) * 2**(-6) = 25.109

   
   
2. Examples of the CDC 7600 floating point word

   The base value in Word 3 will be in the form:

   BITS         1  2     12  13     60
   -----------------------------------
   MEANING   sign  exponent  magnitude
   -----------------------------------
   

   When a number is negative (sign bit on) the entire word will be in 1's complement form. The exponent is biased by 2000 (base 8) for exponents greater than zero, and 1777 (base 8) for exponents less than zero. The binary point for the magnitude is assumed to be at the extreme right, Thus a packed value of 1773 (base 8) would represent an exponent of -4 (base 8). A value of 2005 (base 8) would represent an exponent of +5 (base 8). Values ordinarily are normalized to the left with the first significant digit in the high order position.

   Value	7600 Representation (Octal)
   -2.000	60563777777777777777
   -1.000	60573777777777777777
   0.0	00000000000000000000
   1.000	17204000000000000000
   2.000	17214000000000000000
   3.125	17216200000000000000
   301100.000	17424460260000000000
   11300.000	17355411000000000000

   TABLE 1 - LEVELS AND VARIABLES Level (mb) Variables 1013 slp sst currents 1001 t ddep e u v dd ff 1000 z t ddep e u v dd ff 925 z t ddep e u v omg 850 z t ddep e u v dd ff omg 700 z t ddep e u v dd ff omg 501 1000-500 thick 500 z t ddep e u v dd ff omg 400 z t ddep e u v dd ff 300 z t ddep e u v dd ff omg 250 z t u v dd ff 200 z t u v dd ff omg 150 z t u v dd ff omg 100 z t u v dd ff omg 70 z t u v 50 z t u v dd ff omg 30 z t u v dd ff omg 20 z t u v 10 z t u v dd ff omg 5 z t u v 2 z t u v 1 z t u v 0.4 z t u v **Not all of these are available in each grid type, see inventories.

   TABLE 2 - VARIABLE CODES AND UNITS Variable Code Meaning Units 1 z - geopotential height g.p. centimeters 6 slp - sea level pressure millibars 10 t - temperature Celsius 12 ddep - dewpoint depression Celsius 30 u - u wind component centimeters per second 31 v - v wind component centimeters per second 33 **dd - wind direction degrees / 10 34 **ff - wind force knots 40 omg - vertical motion centimeters per second 45 e - vapor pressure millibars 55 sst5 - sst 5 day mean Celsius 56 ssta - sst anomaly Celsius 57 sst - sea sfc temperature Celsius 58 sst - NCAR yrmo mean sst Celsius 59 sst - NCAR long term mean Celsius **these are in a geographic orientation (whereas the u and v components are positive along the positive i and j directions of the grid)