Open Geospatial Consortium

Submission Date: 2016-03-08

Approval Date:      2018-01-16

Publication Date:      2018-11-27

External identifier of this OGC® document: http://www.opengis.net/doc/IS/CIS-GRIB2/1.0

Internal reference number of this OGC® document:        16-060r2

Version: 1.0

Category: OGC® Implementation Standard

Editor:      Daniel Lee

OGC GML Application Schema – Coverages: GRIB2 Coverage Encoding Profile

Copyright notice

Copyright © 2018 Open Geospatial Consortium

To obtain additional rights of use, visit http://www.opengeospatial.org/legal/

Warning

This document is an OGC Member approved international standard. This document is available on a royalty free, non-discriminatory basis. Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to provide supporting documentation.

Document type:        OGC® Standard

Document subtype:    Profile

Document stage:    Approved for public release

Document language:  English

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i. Abstract

This OGC standard is a profile of the OGC GML Application Schema - Coverages version 1.0 [OC 09-146r2]. That document was renamed "OGC Coverage Implementation Schema (CIS)" for clarification in version 1.1. This standard specifies the usage of the GRIB2 data format for the encoding of OGC coverages. The GRIB2 specification is maintained by the World Meteorological Organization (WMO) and is the standard encoding for the exchange and storage of general regularly distributed information expressed in binary form.

ii. Keywords

The following are keywords to be used by search engines and document catalogues.

ogcdoc, OGC document, ogcdoc, wcs, grib, wmo

iii. Preface

Coverages represent digital geospatial information representing space/time-varying phenomena. OGC Abstract Topic 6 [OGC 07-011] - which is identical to ISO 19123 - defines an abstract model of coverages. Due to the generality of the ISO 19123 model, implementations of the model are not always interoperable. The OGC CIS establishes a concise, interoperable coverage model based on ISO 19123. Coverage instances represented according to the CIS version 1.1 [OGC 09-146r3] may be encoded in GML (based on the Geography Markup Language (GML) version 3.2 [OGC 07-036]) or other suitable ASCII or binary formats.

This CIS profile specifies the usage of the GRIB2 data format for the encoding of OGC coverages. The profile is based on the authoritative format specification available in the WMO Manual on Codes.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. The Open Geospatial Consortium shall not be held responsible for identifying any or all such patent rights.

Recipients of this document are requested to submit, with their comments, notification of any relevant patent claims or other intellectual property rights of which they may be aware that might be infringed by any implementation of the standard set forth in this document, and to provide supporting documentation.

iv. Submitting organizations

The following organizations submitted this Document to the Open Geospatial Consortium (OGC):

Organization name(s)

Deutscher Wetterdienst
Météo-France
rasdaman GmbH
UK Met Office

v. Submitters

All questions regarding this submission should be directed to the editor or the submitters:

Name

Affiliation

Daniel Lee

Deutscher Wetterdienst

Marie-Francoise Voidrot

Météo-France

Peter Baumann

rasdaman GmbH

Chris Little

UK Met Office

1. Scope

This OGC GML Application Schema – Coverages - GRIB2 Coverage Encoding Profile - henceforth abbreviated as "CIS for GRIB2" - specifies an encoding of coverages in the GRIB2 data exchange format.

The GRIB edition 2 (GRIB2) format specification [GRIB2] is designed for the exchange and storage of general regularly distributed information in binary form. GRIB2 is primarily used to represent the outputs of numerical weather prediction models, but is in no way restricted to this domain. GRIB2 was preceded by GRIB edition 0 and GRIB edition 1. The differences between both of these formats, however, are of such structural magnitude that they can be seen as different formats. For this reason, GRIB editions 0 and 1 are outside the scope of this CIS for GRIB2 standard.

A GRIB2 file is composed of one or more GRIB2 messages, which are composed of several sections providing metadata and one or more 2D arrays of values. The values can be mapped from image space to corresponding points in model space. The 2d array of values can have a rectangular spatial shape. However other, non-regular coordinate systems can be used. Additionally, grid points are not necessarily representative of rectangular areas. Some coordinate systems used in GRIB2 represent their data using triangular grid cells (cf. GRIB2 format specification [GRIB2]).

In the near future it is to be expected that a new GRIB edition (GRIB edition 3) is approved by WMO. When the standard has been finalized and agreed upon, an appropriate encoding profile will be created which allows exchange of data encoded in GRIB3 as Coverages.

The only coverage types supported by this standard are cis:GridCoverage, cis:RectifiedGridCoverage, cis:ReferenceableGridCoverage, and any coverage type derived from these two types.

Following the notation of the abstract coverage definition as specified in the CIS [OGC 09-146r2], the rangeType is limited according to the GRIB2 specification.

2. Conformance

This standard defines the following requirements and conformance class:

Standardization targets are coverage instance documents encoded in GRIB2.

Conformance with this standard shall be checked using all the relevant tests specified in Annex A (normative) of this document. The framework, concepts, and methodology for testing, and the criteria to be achieved to claim conformance are specified in the OGC Compliance Testing Policies and Procedures and the OGC Compliance Testing web site [1].

All requirements-classes and conformance-classes described in this document are owned by the standard(s) identified.

3. References

The following normative documents contain provisions that, through reference in this text, constitute provisions of this document. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. For undated references, the latest edition of the normative document referred to applies.

Note

Conformance classes used:

  • gml-coverage

  • multipart

  • special-format

4. Terms and Definitions

This document uses the terms defined in Sub-clause 5.3 of [OGC 06-121r8], which is based on the ISO/IEC Directives, Part 2, Rules for the structure and drafting of International Standards. In particular, the word “shall” (not “must”) is the verb form used to indicate a requirement to be strictly followed to conform to this standard.

For the purposes of this document, the following additional terms and definitions apply.

4.1. raster or image space

Space used to reference the grid point values (cf. GRIB2 format specification [GRIB2]).

4.2. model space

Space used to reference points on the earth (cf. GRIB2 format specification [GRIB2]).

5. Conventions

5.1. Identifiers

The normative provisions in this specification are denoted by the URI

http://www.opengis.net/spec/wcs/grib2/1.0

All requirements and conformance tests that appear in this document are denoted by partial URIs which are relative to this base.

5.2. Namespace prefix conventions

The following namespaces are used in this document. The prefix abbreviations used constitute conventions used here, but are not normative. However, the namespaces to which the prefixes refer are normative.

Table 1. Namespace mappings

Prefix

Namespace URI

Description

xsd

http://www.w3.org/2001/XMLSchema

XML Schema namespace

gml

http://www.opengis.net/gml/3.2

GML 3.2.1

cis

http://www.opengis.net/cis/1.0

GML Application Schema - Coverages 1.0

grib2

http://www.opengis.net/wcs/grib2/1.0

CIS-GRIB2 Encoding Profile 1.0

wcs

http://www.opengis.net/wcs/2.0

WCS 2.0

6. GRIB2 coverage representation requirements class

Requirements class grib2-coverage establishes how coverages are represented in the GRIB2 encoding format. The requirements class further specifies how coverages can be requested, such as from a WCS instance by defining the parameter for GRIB2 features compression. The identifying URL of grib2-coverage is given by http://www.opengis.net/spec/wcs/grib2/1.0/req/grib2-coverage.

6.1. General

The GRIB2 format allows not only the use of controlled vocabulary which are published by the WMO, but also variables defined only locally. The following admonition from the WMO’s Manual on Codes should be reiterated at this juncture:

“…​ the use of Local tables in messages intended for non-local or international exchange is strongly discouraged.”

In this sense, it is possible to encode GRIB2 messages using locally defined variables, but the likelihood that users will be able to interpret these messages is much lower than when the tables published by the WMO are used. If locally defined variables are used to exchange data, however, the tables required for reading the data should be supplied to users in an appropriate fashion.

Requirement 1

http://www.opengis.net/spec/wcs/grib2/1.0/req/grib2-coverage/grib2-specification

A GRIB2 encoded coverage shall follow the GRIB2 specification [GRIB2].

Requirement 2

/req/grib2-coverage/type

A GRIB2 encoded coverage shall be a concrete subclass of type cis:AbstractCoverage.

6.2. Domain

Requirement 3

/req/grib2-coverage/crs

If the coverage instance under test is encoded in a multipart message and its first part consists of a GML document which is a concrete subclass of cis:AbstractCoverage, the coordinate reference system identified by the value of the srsName attribute of the gml:Envelope element of the gml:domainSet element of a GRIB2 encoded coverage shall be the same as the coordinate reference system used in the GRIB2.

Note: The value of the srsName attribute of the Envelope element shall be inherited by all directly expressed geometries (see GML 3.2 [OGC 07-036] clause 9.10), i.e. for all range set values encoded in GRIB2.

Requirement 4

/req/grib2-coverage/pixel-is-area

The domain of a GRIB2 encoded coverage shall respect the coverage’s raster space as defined in the GRIB2 specification [GRIB2], i.e. the grid point position described in code table 3.8 [2] of the GRIB2’s grid definition section.

Note: The definition of grids in GML 3.2 [OGC 07-036] clause 19.2.2 which CIS is based on reads: "When a grid point is used to represent a sample space (e.g. image pixel), the grid point represents the center of the sample space (see ISO 19123:2005, 8.2.2).”

Note: This requirement specifies that the gml:domainSet element shall respect the raster type i.e. it shall include the half pixel border in case of PixelIsArea (see also Figure 1) but not in case of PixelIsPoint. In other words, in case of PixelIsArea the gml:domainSet element is decreased by the half of both gml:offsetVector elements in the gml:lowerCorner coordinate compared to the case of PixelIsPoint. The gml:origin element stays the same independently of the raster space.

6.2.1. Examples

The following XML fragments show the domainSet and the domainSet elements describing the domain of sample coverages respecting the axis ordering of the used CRS:

  • Default axis order (northing, easting) using PixelIsArea:

<gml:boundedBy>
<gml:Envelope srsName="http://www.opengis.net/def/crs/EPSG/0/4326"
srsDimension="2">
<gml:lowerCorner>29.4375 62.4375</gml:lowerCorner>
<gml:upperCorner>70.5625 336.5625</gml:upperCorner>
</gml:Envelope>
</gml:boundedBy>
<gml:domainSet>
<gml:RectifiedGrid dimension="2" gml:id="grid_p">
<gml:limits>
<gml:GridEnvelope>
<gml:low>0 0</gml:low>
<gml:high>657 1377</gml:high>
</gml:GridEnvelope>
</gml:limits>
<gml:origin>
<gml:Point gml:id="grid_origin_p"
srsName="http://www.opengis.net/def/crs/EPSG/0/3857">
<gml:pos>0 0</gml:pos>
</gml:Point>
</gml:origin>
<gml:offsetVector srsName="http://www.opengis.net/def/crs/EPSG/0/3857">
0.0625 0.0625
</gml:offsetVector>
</gml:RectifiedGrid>
</gml:domainSet>

  • Default axis order (northing, easting) using PixelIsPoint:

<gml:boundedBy>
<gml:Envelope srsName="http://www.opengis.net/def/crs/EPSG/0/4326"
srsDimension="2">
<gml:lowerCorner>29.4375 62.4375</gml:lowerCorner>
<gml:upperCorner>70.5625 336.5625</gml:upperCorner>
</gml:Envelope>
</gml:boundedBy>
<gml:domainSet>

... (same as above)

</gml:domainSet>

The simple figure below provides an explanation of the relation between raster or image space and model space in the case of PixelIsArea using the values from the examples above.

Figure 1. Relation between raster or pixel and model space in case of PixelIsArea. Coordinates connected to the grid with two lines represent model space, whereas coordinates connected with one line represent raster space.
       100 110           490
       | 0 |             |
       | | |             |
350 -- +---+---+---+ ... +---+
   0 - | * | * | * | ... | * |
340 -- +---+---+---+ ... +---+
       | * | * | * | ... | * |
       +---+---+---+ ... +---+
       ...
       +---+---+---+ ... +---+ -- 60
       | * | * | * | ... | * | - 29
       +---+---+---+ ... +---+ -- 50
                           | |
                          39

6.3. GRIB2 parameters

6.3.1. Request

The following GRIB2 parameters are defined for requesting GRIB2 encoded coverages, for example via WCS GetCoverage requests.

Note: It is always possible for a given coverage instance to determine its current parameter values, i.e., the parameter values used at creation. Thus, a GRIB2 encoded coverage is self-describing.

Requirement 5

/req/grib2-coverage/parameters-xml

The structure of an XML request requesting a GRIB encoded coverage instance shall be extended as defined in Table 2 and the respective XML Schema being part of this standard.

Requirement 6

/req/grib2-coverage/parameters-kvp

In a GET/KVP GetCoverage request, GRIB2-specific parameters shall adhere to the syntax and semantics defined in Table 2.

Note: This prefix is used as namespace replacement in order to minimize the risk of reusing already defined KVP parameters.

Table 2. Parameters for requesting GRIB2 encoded coverage

Names

Definition

Data type and values

Multiplicity and use

grib2:compression

Compression type according to GRIB2 code table 5.0 “Data representation template number”

Integer between 0 and 65535

Zero or one (optional)

6.3.2. Response

Requirement 7

/req/grib2-coverage/compression

The response to a successful request for a GRIB2 encoded coverage containing a compression GRIB2 parameter shall consist of a GRIB2 encoded coverage using the specified compression.

Note: A server or service not implementing the requested feature shall respond with an exception.

6.3.3. Exceptions

Requirement 8

/req/grib2-coverage/exceptions

When a server or service encounters an error described in column "meaning of exception code" in Table 3, the service shall return the corresponding exception report message with the contents of the locator parameter value as specified in the right column of Table 3 (Exception codes for GRIB2 parameters).

Table 3. Exception codes for GRIB2 parameters

exceptionCode value

HTTP code

Meaning of code

locator value

CompressionNotSupported

404

Server does not support the requested compression

Value of compression parameter

CompressionInvalid

404

Invalid compression requested

Value of compression parameter

7. Media Types for data encodings

Requirement 9

/req/grib2-coverage/mime-type-identifier

GRIB2 encoding of a coverage shall be indicated by the following MIME type identifier:

application/wmo-GRIB2

Note: this MIME type is new and will be registered with IANA.

Annex A: Conformance Class Abstract Test Suite (Normative)

The OGC URI identifier of this conformance class is: http://www.opengis.net/spec/CIS_grib2-coverages/1.0/conf/grib2-coverage

A.1. Prerequisites

Make sure that at least one GRIB2 encoded coverage instance is available. Repeat the tests for each GRIB2 encoded coverage instance available.

A.2. Follow GRIB2 specification

Test identifier

/conf/grib2-coverage/grib2-specification

Requirement

http://www.opengis.net/spec/wcs/grib2/1.0/req/grib2-coverage/grib2-specification

Dependency

http://www.wmo.int/pages/prog/www/WMOCodes/WMO306_vI2/VolumeI.2.html

Test purpose

A GRIB2 encoded coverage shall follow the GRIB2 specification [GRIB2].

Test method

Validate the coverage instance under test against the GRIB2 specification. Test passes if coverage instance is valid according to the GRIB2 specification.

Test type

A.3. Correct coverage type

Test identifier

/conf/grib2-coverage/type

Requirement

/req/grib2-coverage/type

Dependency

http://www.opengis.net/spec/CIS/1.0/conf/gml-coverage

Test purpose

A GRIB2 encoded coverage shall be of type cis:GridCoverage, cis:RectifiedGridCoverage, or cis:ReferenceableGridCoverage, or a subtype thereof.

Test method

If the coverage instance under test is encoded in a multipart message check that its first part consists of a GML document of type cis:GridCoverage, cis:RectifiedGridCoverage, or cis:ReferenceableGridCoverage, or a subtype thereof. Test passes if constraint evaluates to true.

Test type

A.4. Correct URI

Test identifier

/conf/grib2-coverage/uri

Requirement

Dependency

Test purpose

If the usage of URIs is possible, GRIB2 encoding of a coverage shall be indicated by the following URI: http://www.opengis.net/spec/CIS_grib2-coverages/1.0/req/grib2-coverage

Test method

If the coverage instance under test is encoded in a multipart message, check that the xlink:role attribute of the gml:rangeParameters element of the gml:File element of the gml:rangeSet element of its first part has the value http://www.opengis.net/spec/CIS_grib2-coverages/1.0/req/grib2-coverage

Test passes if constraint evaluates to true.

Test type

A.5. Correct MIME type

Test identifier

/conf/grib2-coverage/mime-type-identifier

Requirement

/req/grib2-coverage/mime-type-identifier

Dependency

Test purpose

GRIB2 encoding of a coverage shall be indicated by the following MIME type identifier:

application/wmo-grib2

if the type has already been registered at IANA. Until this time the following MIME type identifier is allowed provisionally:

application/x-grib2

Test method

If the coverage instance under test is encoded in a multipart message, check that the gml:mimeType element of the gml:File element of the gml:rangeSet element of its first part, as well as the Content-Type header of its second part have the value application/x-grib2.

Test passes if constraints evaluate to true.

Test type

A.6. Correct CRS

Test identifier

/conf/grib2_coverages/crs

Requirement

/req/grib2-coverage/crs

Dependency

Test purpose

The coordinate reference system identified by the value of the srsName attribute of the gml:Envelope element of the gml:domainSet element of a GRIB2 encoded coverage shall be the same as the coordinate reference system used in the GRIB2 part.

Test method

If the coverage instance under test is encoded in a multipart message, check that the srsName attribute of the gml:Envelope element of the gml:domainSet element in the first part defines the same coordinate reference system as the one used in the second part, which in case of cis:ReferenceableGridCoverage is the coordinate reference system used for the tiepoints and in the case of cis:GridCoverage is not present.

Test passes if constraint evaluates to true.

Test type

A.7. Correct raster space

Test identifier

/conf/grib2-coverages/pixel-is-area

Requirement

/req/grib2-coverage/pixel-is-area

Dependency

Test purpose

The domain of a GRIB2 encoded coverage shall respect the coverage’s raster space as defined in the GRIB2 specification [GRIB2], i.e. the grid point position described in code table 3.8 of the GRIB2’s grid definition section.

Test method

If the coverage instance under test is encoded in a multipart message, check that the values used in the gml:lowerCorner and gml:upperCorner elements of the gml:Envelope element of the gml:domainSet element of its first part respect the raster space settings used in the second part.

In case of PixelIsArea, type cis:RectifiedGridCoverage, and grid axis parallel to the CRS axis, the difference between the values used in the gml:lowerCorner and gml:Envelope elements shall be the same as the number of pixels defined in the gml:domainSet element multiplied with the respective value of the respective gml:offsetVector element, taking into account the right axis ordering.

In case of PixelIsPoint, type cis:RectifiedGridCoverage, and grid axis parallel to the CRS axis, the difference between the values used in the gml:lowerCorner and gml:Envelope elements shall be the same as the number of pixels defined in the gml:domainSet element minus 1 multiplied with the respective value of the respective gml:offsetVector element, taking into account the right axis ordering.

Test passes if constraint evaluates to true.

Test type

A.8. Parameters XML

Test identifier

/conf/grib2-coverage/parameters-xml

Requirement

/req/grib2-coverage/parameters-xml

Dependency

Test purpose

The structure of an XML request requesting a GRIB2 encoded coverage instance shall be extended as defined in Table 2 and the respective XML Schema being part of this standard.

Test method

Validate the GRIB2 parameters present in the request if an XML request was used to obtain the coverage instance under test.

Test passes if an XML request was used to obtain the coverage instance under test and the GRIB2 parameters are valid according to the XML schema and the coverage instance is encoded as requested.

Test type

A.9. Parameters KVP

Test identifier

/conf/grib2-coverage/parameters-kvp

Requirement

/req/grib2-coverage/parameters-kvp

Dependency

Test purpose

The structure of a KVP request requesting a GRIB2 encoded coverage instance shall be extended as defined in Table 2 where the parameter names shall be prefixed with grib2, for example grib2:compression.

Test method

Validate the GRIB2 parameters present in the request if a KVP request was used to obtain the coverage instance under test.

Test passes if a KVP request was used to obtain the coverage instance under test and the GRIB2 parameters are valid according to Table 2 and the coverage instance is encoded as requested.

Test type

A.10. Compression

Test identifier

/conf/grib2-coverage/compression

Requirement

/req/grib2-coverage/compression

Dependency

Test purpose

The response to a successful request for a GRIB2 encoded coverage containing a compression GRIB2 parameter shall consist of a GRIB2 encoded coverage using the specified compression.

Test method

Check the GRIB2 parameters present in the request used to obtain the coverage instance under test for the presence of the compression parameter. If found and the parameter is valid, verify that the coverage instance is encoded using the requested compression.

Test passes if the compression parameter is present and the coverage instance is encoded as requested or if a HTTP 404 error code with the exceptionCode value CompressionNotSupported is returned.

Test type

A.11. Exceptions

Test identifier

/conf/grib2-coverage/exceptions

Requirement

/req/grib2-coverage/exceptions

Dependency

Test purpose

When a server or service encounters an error described in column "meaning of exception code" in Table 3, it shall return the corresponding exception report message with the contents of the locator parameter value as specified in the right column of Table 3.

Test method

Validate possible exception responses obtained instead of coverage instances when requesting GRIB2 encoded coverages using unsupported and invalid compression parameters.

Test passes if exceptions are using the relevant exceptionCode.

Test type

Annex B: Revision History

Date Release Editor Primary clauses modified Description

2015-10-15

0.0.1

Daniel Lee

All

First draft based on discussions in DWG MetOcean

2015-10-28

0.0.2

Peter Baumann

Abstract, Overview, Domain

Minor corrections

2015-10-29

0.0.3

Daniel Lee

Overview, identification, range

Draft incorporating suggestions gathered in telephone conference with DWG MetOcean

2016-01-20

0.1.0

Daniel Lee

Submitters, submitting organizations

Include new authors of document, reformat for submission to SWG

2016-02-16

0.1.1

Daniel Lee

All

Change references to GMLCOV to proposed CIS

2016-11-11

0.1.2

Carl Reed

Several

Formal corrections

2016-12-16

1.0.0

Daniel Lee

All

Implement suggestions from OAB review

Annex C: Bibliography

1. www.opengeospatial.org/cite
2. http://www.nco.ncep.noaa.gov/pmb/docs/grib2/grib2_table3-8.shtml