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GRIB Format

Edition 1


1. GRIB Overview

The GRIB product is broken down into several subsections:

Tables:

Each section is described below.

1.1 Data Packing

The values saved in a GRIB file are packed.  The actual data value is derived from the following parameters:

  • X = internal value saved in the binary data section.  The values depend on the number of bits per grid value.
  • D = decimal scale factor
  • E = binary scale factor (sign bit plus 15 bit integer)
  • R = reference value

Each gridpoint value (Y)  is determined from the following formula:

Y * 10D = R + (X * 2E)

The reference value (R) uses IBM single precision floating point format.

sAAAAAAA BBBBBBBB BBBBBBBB BBBBBBBB

  • s = sign bit, encoded as 0 means positive, 1 means negative
  • A...A = 7-bit binary integer representing the exponent/characteristic
  • B...B = 24-bit binary integer, the mantissa.

The appropriate formula to recover the value of R is:

R = (-1)s * 2(-24) * B * 16(A-64)


2. Indicator Section

Byte/Octet Description
01-04 "GRIB" - ASCII characters to flag data as GRIB data
05-07 Total number of bytes for all data
08 GRIB edition - currently 1

2. Product Definition Section (PDS)

Octet/Byte

Description

01-03

Length in bytes of PDS

04

Parameter table version number - currently 2 for international exchange

05

Originating Center

06

Generating process or model ID (center dependent)

07

Grid identification - used for fixed grid types, GDS is used for specific grid definition

08

Flag specifying the presence or absence of a GDS or a BMS

09

Parameter and units

10

Level or Layer Type

11-12

Level or Layer values

13 Reference Time - Year of century
14 Month of year
15 Day of month
16 Hour of day
17 Minute of hour

18

Forecast time unit

19

P1 - Period of time (Number of time units). 0 for analysis or initialized analysis.

20

P2 - Period of time (Number of time units) or time interval between successive analyses, successive initialized analyses, or forecasts, undergoing averaging or accumulation.

21

Time range indicator

22-23

Number included in average, when octet 21 (Table 5) indicates an average or accumulation; otherwise set to zero.

24

Number Missing from averages or accumulations.

25

Century of Initial (Reference) time (=20 until Jan. 1, 2001, 21 afterwards)

26

Identification of sub-center

27-28

The decimal scale factor D. A negative value is indicated by setting the high order bit (bit No. 1) in octet 27 to 1 (on).

29-40

Reserved (need not be present)

41-nnn

Reserved for originating center use.


3. Grid Description Section (GDS)

Byte/Octet

Description

01-03

Length in bytes of the GDS

04

NV, the number of vertical coordinate parameters

05

PV, the location (octet number) of the list of vertical coordinate parameters, if present or PL, the location (octet number) of the list of numbers of points in each row (when no vertical parameters are present), if present or (all bits set to 1) if neither are present

06

Data representation type

07-32

Grid description, according to data representation type, except Lambert, Mercator or Space View.

07-42

Grid description for Lambert or Mercator grid

07-44

Grid description for Space View perspective grid

PV

List of vertical coordinate parameters. length = NV x 4 octets; if present, then PL = 4 x NV + PV

PL

List of numbers of points in each row, used for quasi-regular grids. length = NROWS x 2 octets, where NROWS is the total number of rows defined within the grid description

Note: NV and PV relate to features of GRIB not, at present, in use in international exchange. See the WMO Manual on Codes for the descriptions of those features.

PL is used for "quasi-regular" or "thinned" grids; e.g., a lat/lon grid where the number of points in each row is reduced as one moves poleward from the equator. The reduction usually follows some mathematical formula involving the cosine of the latitude, to generate an (approximately) equally spaced grid array. The association of the numbers in octet PL (and following) with the particular row follows the scanning mode specification in Table 8.

3.1 Grid Definition for Latitude/Longitude Grids

Octet/Byte Description
07-08 Ni - No. of points along a latitude circle
09-10 Nj - No. of points along a longitude meridian
11-13 La1 - latitude of first grid point units: millidegrees (degrees x 1000)
values limited to range 0 - 90,000 bit 1 (leftmost) set to 1 for south latitude
14-16 Lo1 - longitude of first grid point units: millidegrees (degrees x 1000)
values limited to range 0 - 360,000 bit 1 (leftmost) set to 1 for west longitude
17 Resolution and component flags
18-20 La2 - Latitude of last grid point (same units, value range, and bit 1 as La1)
21-23 Lo2 - Longitude of last grid point (same units, value range, and bit 1 as Lo1)
24-25 Di - Longitudinal Direction Increment (same units as Lo1) (if not given, all bits set = 1)
26 - 27 Regular Lat/Lon Grid:
Dj - Latitudinal Direction Increment (same units as La1) (if not given, all bits set = 1)
Gaussian Grid:
N - number of latitude circles between a pole and the equator. Mandatory if Gaussian Grid specified
28 Scanning mode flags
29 - 32 Reserved (set to zero)

 Notes:

  • The latitude and longitude of the first and last grid points should always be given, for regular grids.
  • If a quasi-regular grid is to be described, in which all the rows or columns do not necessarily have the same number of grid points, either Ni (octets 7-8) or Nj (octets 9-10) and the corresponding Di (octets 24-25) or Dj (octets 26-27) shall be coded with all bits set to 1 (missing).
  • A quasi-regular grid can be defined only for rows or columns, but not both simultaneously. The first point in each row (column) shall be positioned at the meridian (parallel) indicated in octets 11-16. The grid points shall be evenly spaced in latitude (longitude).
  • For Gaussian grids only the rows can be rendered quasi-regular; the first point shall be located at the meridian given in octets 14-16 and the last point at the meridian given in octets 21-23.

3.2 Grid Definition for Polar Stereographic Grids

Octet

CONTENT & MEANING

07-08

Nx - Number of points along x-axis

09-10

Ny - Number of points along y-axis

11-13

La1 - Latitude of first grid point

14-16

Lo1 - Longitude of first grid point

17

Resolution and component flags

18-20

Lov - The orientation of the grid; i.e., the east longitude value of the meridian which is parallel to the y-axis (or columns of the grid) along which latitude increases as the y-coordinate increases. (Note: The orientation longitude may, or may not, appear within a particular grid.)

21-23

Dx - the X-direction grid length (see note 2)

24-26

Dy - the Y-direction grid length (see note 2)

27

Projection center flag (see note 5)

28

Scanning mode

29 - 32

Set to 0 (reserved)

Notes:

  1. Latitude and longitude are in millidegrees (thousandths)
  2. Grid lengths are in units of meters, at the 60 degree latitude circle nearest to the pole in the projection plane.
  3. Latitude values are limited to the range 0 - 90,000. Bit 1 is set to 1 to indicate south latitude.
  4. Longitude values are limited to the range 0 - 360,000. Bit one is set to 1 to indicate west longitude.
  5. Projection Center Flag - Bit 1 set to 0 if the North pole is on the projection plane. Bit 1 set to 1 if the South pole is on the projection plane.
  6. The first and last grid points may not necessarily be the same as the first and last data points if the bit map section (BMS) is used.
  7. The resolution flag (bit 1 of Table 7) is not applicable.

3.3 Grid Definition for Lambert Conformal, Conic Grids

Octet

CONTENT & MEANING

07-08

Nx - Number of points along x-axis

09-10

Ny - Number of points along y-axis

11-13

La1 - Latitude of first grid point

14-16

Lo1 - Longitude of first grid point

17

Resolution and component flags

18-20

Lov - The orientation of the grid; i.e., the east longitude value of the meridian which is parallel to the y-axis (or columns of the grid) along which latitude increases as the y-coordinate increases. (Note: The orientation longitude may, or may not, appear within a particular grid.)

21-23

Dx - the X-direction grid length (see note 2)

24-26

Dy - the Y-direction grid length (see note 2)

27

Projection center flag (see note 5)

28

Scanning mode

29-31

Latin 1 - The first latitude from the pole at which the secant cone cuts the spherical earth. (See Note 8)

32-34

Latin 2 - The second latitude from the pole at which the secant cone cuts the spherical earth. (See Note 8)

35-37

Latitude of southern pole (millidegrees)

38-40

Longitude of southern pole (millidegrees)

41-42

Reserved (set to 0)

Notes:

  1. Latitude and longitude are in millidegrees (thousandths)
  2. Grid lengths are in units of meters, at the intersection latitude circle nearest to the pole in the projection plane.
  3. Latitude values are limited to the range 0 - 90,000. Bit 1 is set to 1 to indicate south latitude.
  4. Longitude values are limited to the range 0 - 360,000. Bit one is set to 1 to indicate west longitude.
  5. Octet 27:
    Bit 1 set to 0 if the North pole is on the projection plane.
    Bit 1 set to 1 if the South pole is on the projection plane.
    Bit 2 set to 0 if only one projection center used
    Bit 2 set to 1 if projection is bipolar and symmetric
  6. The first and last grid points may not necessarily be the same as the first and last data points if the bit map section (BMS) is used.
  7. The resolution flag (bit 1 of Table 7) is not applicable.
  8. If Latin 1 = Latin 2 then the projection is on a tangent cone.

3.4 Grid Definition for Mercator Grids

Octet CONTENT & MEANING
07-08 Ni - Number of points along a latitude circle
09-10 Nj - Number of points along a longitude meridian
11-13 La1 - Latitude of first grid point
14-16 Lo1 - Longitude of first grid point
17 Resolution and component flags
18-20 La2 - latitude of last grid point
21-23 Lo2 - longitude of last grid point
24-26 Latin - The latitude(s) at which the Mercator projection cylinder intersects the earth.
27 Reserved (set to 0)
28 Scanning mode
29-31 Di - the longitudinal direction increment (see Note 2)
32-34 Dj - the latitudinal direction increment (see note 2)
35-42 Reserved (set to 0)

Notes:

  1. Latitude and longitude are in millidegrees (thousandths)
  2. Grid lengths are in units of meters, at the circle of latitude specified by Latin.
  3. Latitude values are limited to the range 0 - 90,000. Bit 1 is set to 1 to indicate south latitude.
  4. Longitude values are limited to the range 0 - 360,000. Bit one is set to 1 to indicate west longitude.
  5. The latitude and longitude of the last grid point should always be given.
  6. The first and last grid points may not necessarily be the same as the first and last data points if the bit map section (BMS) is used.

3.5 Grid Definition for Space View or Orthographic

Octet

CONTENTS

07-08

Nx - number of points along x axis (columns)

09-10

Ny - number of points along y axis (rows or lines)

11-13

Lap - latitude of sub-satellite point

14-16

Lop - longitude of sub-satellite point

17

Resolution and component flags (Table 7)

18-20

dx - apparent diameter of earth in grid lengths, in x direction

21-23

dy - apparent diameter of earth in grid lengths, in y direction

24-25

Xp - X-coordinate of sub satellite point

26-27

Yp - Y-coordinate of sub-satellite point

28

Scanning Mode (Table 8)

29-31

the orientation of the grid; i.e., the angle in millidegrees between the increasing y axis and the meridian of the sub-satellite point in the direction of increasing latitude (see Note 3).

32-34

Nr - the altitude of the camera from the earth's center, measured in units of the earth's (equatorial) radius (See Note 4).

35-44

reserved

 Notes:

  1. It is assumed that the satellite is at its nominal position, i.e., it is looking directly at its sub-satellite point.
  2. Octet 32-34 shall be set to all ones (missing) to indicate the orthographic view (from infinite distance).
  3. It is the angle between increasing y axis and the meridian 180 degrees east if the sub-satellite point is the North pole; or the meridian 0 degrees, if the sub-satellite point is the south pole.
  4. The apparent angular size of the earth will be given by 2 x asin(1/Nr).
  5. The horizontal and vertical angular resolutions of he sensor (Rx and Ry), needed for navigation equations, can be calculated from the following

Rx = 2 x asin(1/Nr) / dx
Ry = 2 x asin(1/Nr) / dy


4. Bit Map Section (BMS)

This section defines which grid points will be presented in the Binary Data Section.   A 1 value indicates a value will appear.  A 0 value means it will be absent.

Octet

 

01-03

Length in bytes of BMS

04

Number of unused bits at end of Section 3.

05-06

Numeric: if 0: a bit map follows;
otherwise: the numeric refers to a predefined bit map provided by the center

07-nnn

Bit map, zero filled to an even number of octets


5. Binary Data Section (BDS)

This section contains the packed binary data. 

Octet

 

01-03

Length in octets of binary data section

04

Bits 1 through 4: Flag
Bits 5 through 8: Number of unused bits at end of Section 4.

05-06

The binary scale factor (E). A negative value is indicated by setting the high order bit (bit No. 1) in octet 5 to 1 (on).

07-10

Reference value (minimum value); floating point representation of the number.

11

Number of bits into which a datum point is packed

12-nnn

Variable, depending on octet 4; zero filled to an even number of octets.

14

Optionally, may contain an extension of the flags in octet 4.


6. End Section

This is a key sequence used to denote the end of a GRIB product.

Octet  
1-4 "7777"

A. Originating Centers (PDS)

Value Contents
07 US Weather Service - National Met. Center
08 US Weather Service - NWS Telecomms Gateway
09 US Weather Service - Field Stations
34 Japanese Meteorological Agency - Tokyo
52 National Hurricane Center, Miami
54 Canadian Meteorological Service - Montreal
57 U.S. Air Force - Global Weather Center
58 US Navy - Fleet Numerical Oceanography Center
59 NOAA Forecast Systems Lab, Boulder CO
74 U.K. Met Office - Bracknell
85 French Weather Service - Toulouse
97 European Space Agency (ESA)
98 European Center for Medium-Range Weather Forecasts - Reading
99 DeBilt, Netherlands

B. Process/Model ID

NCEP values:

Value Contents
10 Significant Wind Wave model
19 LFM model
25 Snow Cover analysis
39 NGM model
43 Global Optimal Interpolation analysis
44 Sea Surface Temperature analysis
46 Spectral model - 30 wave
53 LFM analysis
64 Regional Optimal Interpolation analysis
69, 77, 81, 96 Aviation (AVN) model
78, 80, 82, 94 Medium Range Forecast (MRF) model
83, 84, 85, 89 Eta model
86 RUC model

C. Grid Indentifiers

International Exchange Grids

Value

Resolution
degree
lonxlat

Area
Coverage

Grid Shape
Row/Columns

Gridpoints

21

5.0 x 2.5

0-180E, 0-90N

37

36 + pole

1333

22

5.0 x 2.5

180W-0, 0-90N

37

36 + pole

1333

23

5.0 x 2.5

0-180E, 90S-0

pole + 37

36

1333

24

5.0 x 2.5

180W-0, 90S-0

pole + 37

36

1333

25

5.0 x 5.0

0-355E, 0-90N

72

18 + pole

1297

26

5.0 x 5.0

0-355E, 90S-0

pole + 72

18

1297

61

2.0 x 2.0

0-180E, 0-90N

91

45 + pole

4096

62

2.0 x 2.0

180W-0, 0-90N

91

45 + pole

4096

63

2.0 x 2.0

0-180E, 90S-0

pole + 91

45

4096

64

2.0 x 2.0

180W-0, 90S-0

pole + 91

45

4096

255

(non-standard grid - defined in the GDS)

D. GDS/BMS Flag

BIT VALUE MEANING
1 0 GDS Omitted
  1 GDS Included
2 0 BMS Omitted
  1 BMS Included
3-8 0 reserved

E. Parameter and Units

VALUE PARAMETER UNITS
000 Reserved  
001 Pressure Pa
002 Pressure reduced to MSL Pa
003 Pressure tendency Pa/s
004    
005    
006 Geopotential m2/s2
007 Geopotential height Gpm
008 Geometric height M
009 Standard deviation of height M
010    
011 Temperature K
012 Virtual temperature K
013 Potential temperature K
014 Pseudo-adiabatic potential temperature K
015 Maximum temperature K
016 Minimum temperature K
017 Dew point temperature K
018 Dew point depression (or deficit) K
019 Lapse rate K/m
020 Visibility M
021 Radar Spectra (1) -
022 Radar Spectra (2) -
023 Radar Spectra (3) -
024    
025 Temperature anomaly K
026 Pressure anomaly Pa
027 Geopotential height anomaly Gpm
028 Wave Spectra (1) -
029 Wave Spectra (2) -
030 Wave Spectra (3) -
031 Wind direction Deg. true
032 Wind speed m/s
033 u-component of wind m/s
034 v-component of wind m/s
035 Stream function m2/s
036 Velocity potential m2/s
037 Montgomery stream function m2/s2
038 Sigma coord. vertical velocity s /s
039 Pressure Vertical velocity Pa/s
040 Geometric Vertical velocity m/s
041 Absolute vorticity /s
042 Absolute divergence /s
043 Relative vorticity /s
044 Relative divergence /s
045 Vertical u-component shear /s
046 Vertical v-component shear /s
047 Direction of current Deg. true
048 Speed of current m/s
049 u-component of current m/s
050 v-component of current m/s
051 Specific humidity kg/kg
052 Relative humidity %
053 Humidity mixing ratio kg/kg
054 Precipitable water kg/m2
055 Vapor pressure Pa
056 Saturation deficit Pa
057 Evaporation kg/m2
058 Cloud Ice kg/m2
059 Precipitation rate kg/m2/s
060 Thunderstorm probability %
061 Total precipitation kg/m2
062 Large scale precipitation kg/m2
063 Convective precipitation kg/m2
064 Snowfall rate water equivalent kg/m2s
065 Water equiv. of accum. snow depth kg/m2
066 Snow depth M
067 Mixed layer depth M
068 Transient thermocline depth M
069 Main thermocline depth M
070 Main thermocline anomaly M
071 Total cloud cover %
072 Convective cloud cover %
073 Low cloud cover %
074 Medium cloud cover %
075 High cloud cover %
076 Cloud water kg/m2
077    
078 Convective snow kg/m2
079 Large scale snow kg/m2
080 Water Temperature K
081 Land-sea mask Fraction
  (1=land;0=sea) (see note)  
082 Deviation of sea level from mean M
083 Surface roughness M
084 Albedo %
085 Soil temperature K
086 Soil moisture content kg/m2
087 Vegetation %
088 Salinity kg/kg
089 Density kg/m3
090 Water run off kg/m2
091 Ice concentration Fraction
  (ice=1;no ice=0)(see note)  
092 Ice thickness M
093 Direction of ice drift deg. true
094 Speed of ice drift m/s
095 u-component of ice drift m/s
096 v-component of ice drift m/s
097 Ice growth rate m/s
098 Ice divergence /s
099 Snow melt kg/m2
100 Significant height of combined wind m
  waves and swell  
101 Direction of wind waves deg. true
102 Significant height of wind waves m
103 Mean period of wind waves s
104 Direction of swell waves deg. true
105 Significant height of swell waves m
106 Mean period of swell waves s
107 Primary wave direction deg. true
108 Primary wave mean period s
109 Secondary wave direction deg. true
110 Secondary wave mean period s
111 Net short-wave radiation (surface) W/m2
112 Net long wave radiation (surface) W/m2
113 Net short-wave radiation (top of atmos.) W/m2
114 Net long wave radiation (top of atmos.) W/m2
115 Long wave radiation W/m2
116 Short wave radiation W/m2
117 Global radiation W/m2
118    
119    
120    
121 Latent heat net flux W/m2
122 Sensible heat net flux W/m2
123 Boundary layer dissipation W/m2
124 Momentum flux, u component N/m2
125 Momentum flux, v component N/m2
126 Wind mixing energy J
127 Image data  
128-254 Reserved for use by originating center)  
255 Missing

F. Level or Layer

Octet 10 - Type of level/layer Octet 11 - Value 1 Octet 12 - Value 2
00 - reserved
01 - surface of earth including sea surface
02 - cloud base
03 - cloud top
04 - freezing level (0C)
05 - adiabatic condensation level
06 - maximum wind level
07 - tropopause level
08 - nominal top of atmosphere
09 - sea bottom
10-99 reserved  
100 - isobaric level pressure in hectoPascals (hPa)
101 - layer between two isobaric levels pressure of top (kPa) pressure of bottom (kPa)
102 - mean sea level 0 0
103 - fixed height level above mean sea level (MSL) height in meters
104 - layer between two height levels above MSL height of top (hm) height of bottom (hm)
105 - fixed height above ground level height in meters
106 - layer between two height levels above ground height of top (hm) height of bottom (hm)
107 - sigma level sigma value in 1/10000
108 - layer between two sigma levels value at top in 1/100 value at bottom in 1/100
109 - hybrid level level number
110 - layer between two hybrid levels level number of top level number of bottom
111 - depth below surface centimeters
112 - layer between two levels below surface depth of top (cm) depth of bottom (cm)
113 - isentropic level potential temperature (K)
114 - layer between two isentropic levels 475-theta of top (K) 475-theta of bottom (K)
121 - layer between two pressure levels 1100-pressure of top (hPA) 1100-pressure of bottom (hPa)
125 - height level height (cm)
128 - layer between two sigma levels 1.1-sigma of top in 1/1000 1.1-sigma of bottom in 1/1000
141 - layer between two pressure levels pressure of top (kPa) 1100-pressure of bottom (hPa)
160 - depth below sea level depth (m)
200 - entire atmosphere
201 - entire ocean

G. Forecast Time Units

VALUE TIME UNIT
0 minute
1 hour
2 day
3 month
4 year
5 decade
6 normal (30 years)
7 century
8-253 reserved
254 second

H. Time Range Indicator

VALUE MEANING

00

Forecast product valid at reference time + P1 P1>0), or uninitialized analysis product for reference time (P1=0) or image product for reference time (P1=0)

01

Initialized analysis product for reference time (P1=0).

02

Product with a valid time ranging between reference time + P1 and reference time + P2

03

Average (reference time + P1 to reference time + P2)

04

Accumulation (reference time + P1 to reference time + P2). Product considered valid at reference time + P2

05

Difference(reference time + P2 minus reference time + P1) product considered valid at reference time + P2

06-09

reserved

10

P1 occupies octets 19 and 20; product valid at reference time + P1

11-50

reserved

51

Climatological Mean Value: multiple year averages of quantities which are themselves means over some period of time (P2) less than a year. The reference time (R) indicates the date and time of the start of a period of time, given by R to R + P2, over which a mean is formed; N indicates the number of such period-means that are averaged together to form the climatological value, assuming that the N period-mean fields are separated by one year. The reference time indicates the start of the N-year climatology. N is given in octets 22-23 of the PDS.

If P1 = 0 then the data averaged in the basic interval P2 are assumed to be continuous, i.e., all available data are simply averaged together.

If P1 = 1 (the units of time - octet 18, code table 4 - are not relevant here) then the data averaged together in the basic interval P2 are valid only at the time (hour, minute) given in the reference time, for all the days included in the P2 period. The units of P2 are given by the contents of octet 18 and Table 4.

52-112

reserved

113

Average of N forecasts (or initialized analyses); each product has forecast period of P1 (P1=0 for initialized analyses); products have reference times at intervals of P2, beginning at the given reference time.

114

Accumulation of N forecasts (or initialized analyses); each product has forecast period of P1 (P1=0 for initialized analyses); products have reference times at intervals of P2, beginning at the given reference time.

115

Average of N forecasts, all with the same reference time; the first has a forecast period of P1, the remaining forecasts follow at intervals of P2.

116

Accumulation of N forecasts, all with the same reference time; the first has a forecast period of P1, the remaining follow at intervals of P2.

117

Average of N forecasts, the first has a period of P1, the subsequent ones have forecast periods reduced from the previous one by an interval of P2; the reference time for the first is given in octets 13-17, the subsequent ones have reference times increased from the previous one by an interval of P2. Thus all the forecasts have the same valid time, given by the initial reference time + P1.

118

Temporal variance, or covariance, of N initialized analyses; each product has forecast period P1=0; products have reference times at intervals of P2, beginning at the given reference time.

119 -122

reserved

123

Average of N uninitialized analyses, starting at the reference time, at intervals of P2.

124

Accumulation of N uninitialized analyses, starting at the reference time, at intervals of P2.

125-254

reserved

I. Data Representation Type

VALUE MEANING
00 Latitude/Longitude Grid also called Equidistant Cylindrical or Plate Carree projection grid
01 Mercator Projection Grid
02 Gnomonic Projection Grid
03 Lambert Conformal, secant or tangent, conical or bipolar (normal or oblique) Projection Grid
04 Gaussian Latitude/Longitude Grid
05 Polar Stereographic Projection Grid
06-12 reserved - see Manual on Codes
13 Oblique Lambert conformal, secant or tangent, conical or bipolar, projection
14 - 49 reserved - see Manual on Codes
50 Spherical Harmonic Coefficients
51 - 89 reserved - see Manual on Codes
90 Space view perspective or orthographic grid
91 - 254 reserved - see Manual on Codes

J. Resolution and Component Flags

Bit

Value

Meaning

1

0

Direction increments not given

 

1

Direction increments given

2

0

Earth assumed spherical with radius = 6367.47 km

 

1

Earth assumed oblate spheroid with size

   

as determined by IAU in 1965:

   

6378.160 km, 6356.775 km, f = 1/297.0

3-4

 

reserved (set to 0)

5

0

u- and v-components of vector quantities resolved relative to easterly and northerly directions

 

1

u and v components of vector quantities resolved relative to the defined grid in the direction of increasing x and y (or i and j) coordinates respectively

6-8

 

reserved (set to 0)

 Note: If the GDS is not included in a message then any wind components are assumed to be resolved relative to the grid specified in the PDS with u and v defined as positive in the direction of increasing x and y (or i and j) coordinates respectively.

K. Scanning Mode Flag

BIT VALUE MEANING
1 0 Points scan in +i direction
  1 Points scan in -i direction
2 0 Points scan in -j direction
  1 Points scan in +j direction
3 0 Adjacent points in i direction are consecutive (FORTRAN: (I,J))
  1 Adjacent points in j direction are consecutive (FORTRAN: (J,I))

 Note: i direction is defined as west to east along a parallel of latitude, or left to right along an x axis. j direction is defined as south to north along a meridian of longitude, or bottom to top along a y axis.

L. BDS Flag

BIT VALUE MEANING
1 0 Grid point data
  1 Spherical Harmonic Coefficients
2 0 Simple packing
  1 Second order ("Complex") Packing
3 0 Original data were floating point values
  1 Original data were integer values
4 0 No additional flags at octet 14
  1 Octet 14 contains flag bits 5 - 12

 The following gives the meaning of the bits in octet 14 ONLY if bit 4 is set to 1. Otherwise octet 14 contains regular binary data.

5   Reserved (set to 0)
6 0 Single datum at each grid point
  1 Matrix of values at each grid point
7 0 No secondary bit maps
  1 Secondary bit maps present
8 0 Second order values have constant width
  1 Second order values have different widths
9-12   Reserved (set to 0)

 Notes:

  1. Bit 3 is set to 1 to indicate that the original data were integers; when this is the case any non-zero reference values should be rounded to an integer value prior to placing in the GRIB BDS.
  2. Bit 4 is set to 1 to indicate that bits 5 to 12 are contained in octet 14 of the data section.
  3. Although GRIB is not capable of representing a matrix of data values at each grid point, the meaning of bit 6 is retained in anticipation of a future capability.
  4. When secondary bit maps are present in the data (used in association with second order packing) this is indicated by setting bit 7 to 1.
  5. When octet 14 contains the extended flag information, octets 12 and 13 will also contain "special" information; the actual data will begin in a subsequent octet. See above.