An Introduction to GRIB2
Simon Elliott
EUMETSAT
simon.elliott@eumetsat.int
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Saturday Afternoon Agenda
• Background (i.e. my background!)
• Overview (codes, GRID, GRIB, GRIB2)
• Code Structure (Sections like others, iterations of
sections)
• GRIB2 Code Templates (what kinds are there …
examples)
• Tables (Template tables, code and flag tables,
examples)
• Identification of parameters (?)
• GRIB2 compression (methods, explanations of some)
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Sunday Morning Agenda
• Procedure for code form update (addns to tables,
addition of tables/templates, new editions)
• Changes not requiring code change (adding
table/template entries for decoders … down stream
code change)
• Changes requiring code change (always for data
provider, plus new editions)
• Case studies of Cloud Mask and Precipitation Data
(bespoke Encoder and Decoder, example product)
• GRIB2 Interface for NMC (example of NCEP code)
• Review/Discussion
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Overview
• Intention is to share information efficiently and
unambiguously
• Data should be accessible by anyone, not encrypted
• Format should allow all information to be included (not
rounded or skipped)
• Format should allow addition of new data types
• WMO “codes” developed per data type, TEMP, SYNOP,
SATOB, et c.
FM 35 TEMP
FM 42 AMDAR
FM 41 CODAR
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FM 13 SHIP
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FM 88 SATOB
FM 87 SARAD
FM 86 SATEM
FM 85 SAREP
GRID, GRIB, GRIB2
• GRID was developed for gridded data (forecast, analysis)
– ASCII -> human readable?
– ASCII -> big files, slow transfer
– Symbolic letters and code tables: F1F2NNN in Section 0
• F1F2 is originating centre as per C-1,
• NNN is catalogue number of grid used by centre
• GRIB introduced for binary exchange (storage and
transmission efficiency).
– Edition 0, 1985
– Edition 1, 1990
• GRIB Edition 2 (i.e. GRIB2) because Code Table 2 was full,
EPS data, ...
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GRIB1 structure
Section 0
Indicator section: “GRIB”
Section 1
Product definition section: unique standard
template
Section 2
Grid description section: one of the standard
templates describing a type of grid
Section 3
Bit map section
Section 4
Binary data section
Section 5
End section: “7777”
GRIB uses the concept of Template:
« Description of the standardized layout of a set
of data items »
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GRIB usage
•
•
•
•
On the GTS
By the WAFS Centres (« ICAO » products)
For archiving of fields
on MDD
Main limitation is:
• One parameter on one level for one grid per
GRIB field
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GRIB1 weaknesses
Transmission and archiving of:
• Spectral data
• Multi-dimension data
• Long-range and climate products
• Ensemble products (EPS)
Also:
• No convention for missing data
• IEEE not used for floating point data
• No support for small time steps
• No cross-sections, no time-sections
• No Hovmöller Diagrams (ex. Time-longitude)
• Limited support for images
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Need for “object oriented” GRIB2
(more modules = modularity)
• Modularity: code and parameter tables
referred to through templates
• Flexibility: new tables and templates can be
added
• ALSO:
– All GRIB1 fields can be described in GRIB2
– More compression schemes (e.g.
introduction of JPEG 2000 and PNG)
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GRIB2 calendar
•
•
•
•
•
Experimental GRIB2 presented at CBS 98
Finalized in Spring 2000
Approved by CBS in Autumn 2000
Operational in November 2001
First products in 2003
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Generic structure
Table
driven
codes
generally
have this
structure

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• Identification: GRIB/BUFR/CREX
• Header: Date, time, originator,
table versions ...
• Optional section: Metadata
(potentially XML), private data …
• Data description: What sort of data
follows
• Actual data: here
• Closure: “7777”
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GRIB2 structure
•
•
•
•
•
•
•
•
•
Section 0 Indicator section: “GRIB”
Section 1 Identification section: data characteristics
Section 2 Optional (local) section: anything or nothing
Section 3 Grid definition section: geometry of grid used,
potential reference to pre-defined grid
Section 4 Product definition section: description of data
type, e.g. satellite data from spectral bands xx, yy, zz
Section 5 Data representation section: packing method
used for data, reference value, scale factor et c.
Section 6 Bit map section: data present indicators (if used)
Section 7 Data section: data values themselves
Note: Sections 1 to 7 start with
Section 8 End section: “7777”
section length and number
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Section 0
Section 0: Indicator section
–
–
–
–
–
Octets 1 - 4:
Octets 5 - 6:
Octet 7:
Octet 8:
Octets 9 - 16:
“GRIB” in ASCII (i.e. 71 82 73 66)
Reserved (normally set to 0)
Discipline (CT 0.0, e.g. 3 for space)
GRIB edition number (2)
Total length in octets
Messages of length up to ~18x1018 (or 264 - 1)
bytes, i.e. ~18,000,000 Tb
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Section 1
Section 1 Identification section
–
–
–
–
–
Octets 1 - 4:
Octet 5:
Octets 6 - 9:
Octets 10 - 11:
Octet 12:
time)
– Octets 13 - 19:
– Octet 20:
operational data)
– Octet 21:
observations)
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Section length
Section number (1)
Originating centre and sub-centre
Master and local table version numbers
Time significance (CT 1.2, e.g. 3 for observation
Date / time
Production status of data (CT 1.3, e.g. 0 for
Type of data (CT 1.4, e.g. 7 for processed radar
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Section repetition
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Repeatable
Section 0 (Indicator)
Section 1 (Identifier)
Section 2 (Local use)
Section 3 (Grid)
Section 4 (Product)
Section 5 (Data Repn)
Section 6 (Bit map)
Section 7 (Data)
Section 8 (Ending)
Repeatable
•
•
•
•
•
•
•
•
•
Repeatable
GRIB2 allows some groups of sections to be repeated for
efficiency (not in GRIB1, one field per message).
Non-repeated sections stay in effect
Section repetition (example 1)
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Section 0 (Indicator)
Section 1 (Identifier)
Section 3 (Grid)
Section 4 (Product)
Section 5 (Data Repn)
Section 6 (Bit map)
Section 7 (Data)
Section 4 (Product)
Section 5 (Data Repn)
Section 6 (Bit map)
Section 7 (Data)
Section 4 (Product)
Section 5 (Data Repn)
Section 6 (Bit map)
Section 7 (Data)
Section 8 (Ending)
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0600Z
1200Z
1800Z
Forecast from same centre on
same grid for same parameter
at same level but from but for
different validity times
Validity time is in Section 4
(Product Definition)
Section 4 to 7 repeated in
message
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Section repetition (example 2)
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Section 0 (Indicator)
Section 1 (Identifier)
Section 3 (Grid)
Section 4 (Product)
Section 5 (Data Repn)
Section 6 (Bit map)
Section 7 (Data)
Section 3 (Grid)
Section 4 (Product)
Section 5 (Data Repn)
Section 6 (Bit map)
Section 7 (Data)
Section 3 (Grid)
Section 4 (Product)
Section 5 (Data Repn)
Section 6 (Bit map)
Section 7 (Data)
Section 8 (Ending)
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GRID A
GRID B
GRID C
Forecast from same centre for
same validity time for same
parameter at same level but
from but on different grids
Grid is defined in Section 3 (Grid
Definition Section)
Section 3 to 7 repeated in
message
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Grid definition templates
• Grid definition is in Section 3 (GRIB1, Section 2)
• Can refer to centre’s own grid or use Grid Definition
Template (GDT) to specify details
• Many GDTs have been developed, e.g.
–
–
–
–
3.0: Latitude/Longitude (or equidistant cylindrical, or Plate Carrée)
3.1: Rotated Latitude/Longitude (or equidistant cylindrical, or Plate Carrée)
3.2: Stretched Latitude/Longitude (or equidistant cylindrical, or Plate Carrée)
3.3: Stretched and Rotated Latitude/Longitude (or equidistant cylindrical, or
Plate Carrée)
– 3.20: Polar Stereographic
– 3.90: Space view perspective, or orthographic
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A grid definition template example
GDT 3.90 - Space view perspective or orthographic
Floating point numbers stored in 5 bytes as scale factor
(one byte) and scaled value (4 byte IEEE floating point)
Octet
No.
15
Contents
Shape of Earth (CT 3.2: Spherical, oblate sphere, ICAO
shape)
16-20 Radius of spherical Earth
21-30 Major and minor radii of oblate spherical Earth
31-38 Number of pixel columns and rows in grid
39-46 Latitude and longitude of sub-satellite point
47
Resolution and component flags (FT 3.3: Reference
direction for vector components, grid or east/north)
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GDT example continued
Octet Contents
No.
48-55 Apparent diameter of Earth in each direction
56-63 Co-ordinates of sub-satellite point
64
Scanning mode (FT 3.4: location of consecutive scan point
and lines, e.g. 1st row in –x dirn, all scan rows in same dirn)
65-68 Orientation of grid (i.e. skeweness relative to longitude
meridien)
69-72 Height of camera in equatorial radii units, scaled by 106
73-80 Co-ordinates of origin of image
Repeating sections 4 to 7 means this information need not be
repeated for multiple fields in one GRIB2 message
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Product definition templates
• Product definition is in Section 4 (GRIB1, Section 1)
• Many PDTs have been developed, e.g.
– 4.0: Analysis or forecast on horizontal level or layer at a point in time
– 4.1 to 4.4: Various information pertaining to ensemble forecast
systems
– 4.5: Probability forecast on horizontal level or layer at a point in time
– 4.7: Analysis or forecast error on horizontal level or layer at a point in
time
– 4.20: Radar products
– 4.30: Satellite products
• Hybrid sigma levels can be specified in Section 4
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A product definition template
example
PDT 4.30 - Satellite products
Octet
No.
10
Contents
13
Parameter category (CT 4.1: For space products, i.e.
discipline 3, 0 = image, 1 = data are in physical units)
Parameter number (CT 4.2: radiance, albedo, brightness
temperature, skin temperature, cloud mask, …)
Type of generating process (CT 4.3: analysis, forecast,
observation, …)
Observation generating process identifier (locally defined)
14
Number of contributing spectral bands, NB
11
12
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PDT example continued
Octet Contents
No.
15-16 Satellite series of band 1 (defined by originating centre)
17-18 Satellite number of band 1 (defined by originating centre)
19
Instrument type of band 1 (defined by originating centre)
20-24 Central wave number of band 1 (m-1)
Note: Octets 25 to 24 are given for band 1, but are repeated for
each contributing band up to NB, as specified in octet 14
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Code table 4.3: Type of generating
process
Code figure
0
1
2
3
4
5
6
7
8
192 - 254
255
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Meaning
Analysis
Initialisation
Forecast
Bias corrected forecast
Ensemble forecast
Probability forecast
Forecast error
Analysis error
Observation
Reserved for local use
Missing
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Data representation templates
• Data representation is in Section 5 (GRIB1, within Section 4)
• Some DRTs have been developed, e.g.
–
–
–
–
–
–
5.0: Grid point data - simple packing
5.1: Matrix values at grid point - simple packing
5.2: Grid point data - complex packing
5.3: Grid point data - complex packing and spatial differencing
5.50: Spectral data - simple packing
5.51: Spherical harmonics data - complex packing
• Section 5 also gives total number of data values to be found
in Section 7
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A data representation template
example
DRT 5.0 - Grid point data - simple packing
Octet Contents
No.
12-15 Reference value, R (IEEE floating point number)
16-17 Binary scale factor, E
18-19 Decimal scale factor, D
20
Number of bits used for each packed value
21
Type of original field values (CT 5.1: Floating point or
integer)
IEEE floating point numbers in 32 bits:
seeeeeee emmmmmmm mmmmmmmm mmmmmmmm
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Simple and complex packing
• Simple packing
– Y x 10D = R + (X1 + X2) x 2E
– Y is original value, D is decimal scale, R is reference, E is binary
scale, X1 is 0, and X2 is encoded value
– In template R, E, D and bits per value are stored
– Field values follow sequentially
• Complex packing (intended to decrease message size)
– Y x 10D = R + (X1 + X2) x 2E
– R, E, D are as for simple packing
– X1 is reference for group, X2 is scaled value in group (X1
removed)
– Data are split into groups with similar values per group
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Use of D and E
D
0
-1
1
0
-1
-2
1
0
-1
-2
0
E
0
-3
4
1
-2
-5
5
2
-1
-4
3
Increment
1.000
1.250
1.600
2.000
2.500
3.125
3.200
4.000
5.000
6.250
8.000
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DD-1
Increment  Increment x 10
Here R = 0, X1 = 0, X2 = 1
Increment = Y = original value
D
-1
-2
0
-1
-2
-3
0
-1
-2
-3
-1
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E
0
-3
4
1
-2
-5
5
2
-1
-4
3
Increment
10.000
12.500
16.000
20.000
25.000
31.250
32.000
40.000
50.000
62.500
80.000
R, E and D selection example
Temperature on balcony: -15.0 to 35.0, ±0.2
• Set D to give increments in range 1 - 10
– D = 1, i.e scale by 10
– data are now: -150 to 350, ±2
• Set R to minimum
– R = -150.0 (IEEE floating point)
– data are now: 0 to 500, ±2
• Set E to give required precision
– E = 1, i.e. data are in increments of 2
• Set bit width to give required range
– Required range = scaled range / increment
– Required range = 500 / 2 = 250 so 8 bits
R = -150, E = 1, D = 1, bit width = 8
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Spatial differencing
• Saves space for data which vary smoothly (order 1), or
data whose variation varies smoothly (order 2), et c.
• For order 1, data values, f, define new values, g:
– g1 = f1, g2 = f2 - f1, …, gn = fn - fn-1
– 1, 2, 3, 5, 7, 8, 9, 11 becomes 1, 1, 1, 2, 2, 1, 1, 2
• For order 2, g values are replaced by h:
– h1 = f1, h2 = f2, h3 = g3 - g2, …, hn = gn - gn-1
– 1, 2, 4, 10, 17, 27, 40, 57 becomes 1, 2, 1, 4, 1, 3, 3, 4
• Minimum value is subtracted to keep values positive
Order 1
Complex packing
4 groups
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Order 2
Complex packing
3 groups
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Section 6
Section 6: Bit map section
–
–
–
–
Octets 1 - 4:
Octet 5:
Octet 6:
Octets 7 - xx:
Length of section
Section number (6)
Bit map indicator (CT 6.0)
Bit map (if present)
Code Table 6.0: Bit map section
Code figure
0
1 - 253
254
255
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Meaning
Bit map is present and follows in this section
Use bit map predefined by originating centre
Re-use previously defined bit map
Missing (no bit map present)
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Bit map case study
With bit map
No data
No bit map
bit map
(2000 x 2000 x 1)
= 4,000,000 bits
2000 x 2000 x 8 2000
= 32,000,000 bits
2000
Data (8 bits per pixel)
data
( x 1000 x 1000 x 8)
 25,130,000 bits
total
 29,130,000 bits
Conclusion: bit map may not always save space, but if more
than two bits per data point (typically true) it usually will …
consider cloud top height data, space corners, et c.
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Updates not requiring code change
• Type of changes concerned
– Addition of code and or flag table entries
• Agreement and implementation
– “Fast Track” can be used
– Table entries can be approved for pre-operational use within a few
months of consideration and successful validation
– Full formal CBS approval follows, available around 2 years later
• Application updates
–
–
–
–
Encoder needs new code / flag table available
Encoder software needs updating to insert field
Decoder needs new code / flag table available
Down-stream application (beyond decoder) needs to be updated to
process new field
– Decoder software can remain unchanged
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Updates requiring code change
• Type of changes concerned
– Changes to code structure (new edition), or new GDT, PDT or DRT
• Agreement and implementation
– ET/DRC considers a request and proposes update
– Multilateral validation of proposed update
– Full formal CBS approval follows, available around 2 to 4 years later
• Application updates
–
–
–
–
Encoder needs new software and /or template available
Encoder software needs updating accordingly
Decoder needs new software and /or template available
Down-stream application (beyond decoder) needs to be updated to
process new data
• Long lead time necessary … consider PUMA example
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Interfacing to third party software
• Use of third party generic software simplifies
implementation
• NOAA provides GRIB2 software for free download (look at
http://www.nws.noaa.gov/tdl/iwt)
• ECMWF provides GRIBEX software - will be / is being
updated for GRIB2 (look at
http://www.ecmwf.int/products/data/software/grib2.html)
• Typically software package is set of functions compiled into
a library
• User’s application sets up required parameters and calls
function from library, which are referenced at run time
• Specific (not generic) encoders and decoders are quite
simple but less flexible
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Some GRIB2 data from EUMETSAT
• Cloud mask, CLM, every 15 minutes, ~3km resolution, via
EUMETCast or archive request.
• Cloud top height, CTH, every 60 minutes, ~9km
resolution, via EUMETCast or archive request.
• Cloud analysis image, CLAI, every 180 minutes, ~9km
resolution, via EUMETCast or archive request.
• Clear sky reflectance map, CRM, twice per week, ~3km
resolution, via EUMETCast or archive request.
• Multi-sensor precipitation estimate, MPE, every 30
minutes, ~5km resolution, via web site.
• Fire detection, FIR, every 15 minutes, ~3km resolution, via
anonymous FTP (evolving algorithm).
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Case study A, precipitation data
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Case study A, precipitation data
• GRIB2 encoded file
• Specific decoder program for
GRIB2 precipitation data
• Decoder output
• Dump of GRIB2 file
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Case study B, cloud mask data
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Case study B, cloud mask data
• Original data to be encoded
• GRIB2 encoded file
• Specific en/decoder program for
EUMETSAT cloud mask data
• Decoder output
• Dump of GRIB2 file
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Case study C, cloud analysis image
data
GRIB2
Cloud
analysis
image
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Case study C, cloud analysis image
data
• GRIB2 encoded file
• Specific en/decoder program for
EUMETSAT cloud mask data
• Bit map used
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Case study D, cloud top height data
GRIB2
Cloud
top
height
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Case study D, cloud top height data
• GRIB2 encoded file
• Specific en/decoder program for
EUMETSAT cloud mask data
• Bit map used
• Two repeats of Sections 4, 5,
6 and 7
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Sources of GRIB2 information
The information used in preparing this
presentation is based on:
Guide to WMO Table Driven Code Form Used for the
Representation and Exchange of Regularly Spaced
Data in Binary Form: FM 92-XII GRIB
Written by Dr Cliff Dey
&
WMO Manual on Codes, WMO Publication No. 306,
Vol. 1, Part B
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