The OGC CF-netCDF data model supports multi-dimensional gridded data and multi-dimensional multi-point data, representing space and time-varying phenomena. In particular, this extension standard is limited to multi-point, and regular and warped grids.

This standard specifies the CF-netCDF data model encoding using the OGC GML 3.2.1 coverage application schema, as well as CF-netCDF data exchange format and protocol encoding.

This standard specifies: (a) the CF-netCDF GML encoding to be used by OGC standards; (b) the CF-netCDF data format exchanged using OGC standards; (c) the Internet protocol characteristics to effectively exchange CF-netCDF data.

As per the GML 3.3. standard, GML 3.3 imports the 3.2 schema. The canonical location of the 3.2 all components schema document for 3.3 is http://schemas.opengis.net/gml/3.2.1/gml.xsd

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

ogcdoc, netcdf, cf-netcf, encoding extension, multi-dimensional, gml, data format.

This standard describes an extension of the CF-netCDF 3.0 data model [OGC 11-165r02].

This extension is required to encode CF-netCDF 3.0 datasets using the “OGC Implementation Schema for Coverages” specification [OGC 09-146r2] and to exchange CF-netCDF datasets.

Suggested additions, changes, and comments on this draft standard are welcome and encouraged. Such suggestions may be submitted by email message or by making suggested changes in an edited copy of this document.

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.

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

The University Corporation for Atmospheric Research (UCAR);

The Italian National Research Council, Institute of Atmospheric Pollution Research (CNR-IIA, Florence Dept.).

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

1          Scope

This document specifies the CF-netCDF model encoding using the GML 3.2.1 Application Schema –Coverage entities. Besides, it specifies the data format and Internet protocol characteristics to serve CF-netCDF data in an effective and usable way.

GML 3.2.1 has not been deprecated. As per the GML 3.3. standard, GML 3.3 imports the 3.2 schema. The canonical location of the 3.2 all components schema document for 3.3 is http://schemas.opengis.net/gml/3.2.1/gml.xsd.

This CF-netCDF 3.0 extension allows discovery and access services to exchange CF-netCDF data –e.g. encoded in GML. In fact, this specification introduces a set of requirements which a WxS implementation must fulfil to describe and exchange CF-netCDF 3.0 datasets from server to client using the coverage extension of the GML 3.2.1 encoding format.

This standard specifies: (a) the CF-netCDF GML encoding to be used by OGC standards; (b) the CF-netCDF data format exchanged using OGC standards; (c) the Internet protocol characteristics to effectively exchange CF-netCDF data.

CF-netCDF encoding format is netCDF conforming to the Climate and Forecast (CF) conventions (CF-netCDF). It consists of either a binary, XML, or ASCII format which encodes the CF-netCDF data model.

CF-netCDF 3.0 has emerged as a well-supported data model and encoding for several different domains, including: atmospheric science, oceanography, climatology, meteorology, hydrology, etc.

This specification deals with multi-dimensional discrete coverage data and multi-dimensional multi-point data.

2          Conformance

Standardization target is the encoding of netCDF-CF datasets returned by any WxS implementation (currently: servers).

In particular, this standard defines three standardization targets: (a) CF-netCDF GML implementations (currently encodings); (b) CF-netCDF data format (currently encodings); (c) CF-netCDF data format (currently encodings) returned by multiple part messages.

This document establishes three requirements classes, one for each standardization target:

1.     CF-netCDF-1.6 GML encoding, of  http://www.opengis.net/spec/netCDF_data-model/req/CF-netCDF-1.6-GML-encoding -for the standardization target (a);

2.     CF-netCDF-1.6 data format, of    http://www.opengis.net/spec/netCDF_data-model/req/CF-netCDF-1.6-Data-format -for the standardization target (b);

3.     CF-netCDF-1.6 multipart data encoding, of    http://www.opengis.net/spec/netCDF_data-model/req/CF-netCDF-1.6-Multipart-encoding -for the standardization target (c);

and three corresponding pertaining conformance class, CF-netCDF-1.6, with URIs:

1.     http://www.opengis.net/spec/netCDF_data-model/conf/CF-netCDF-1.6-GML-encoding

2.     http://www.opengis.net/spec/netCDF_data-model/conf/CF-netCDF-1.6-Data-format

3.     http://www.opengis.net/spec/netCDF_data-model/req/CF-netCDF-1.6-Multipart-encoding

Requirements and conformance test URIs defined in this document are relative to http://www.opengis.net/spec/netCDF_data-model/.

Annex A lists the conformance tests which shall be exercised on any software artifact claiming to implement an CF-netCDF 3.0 extension for GML 3.2.1 encoding.

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.

OGC 10-090, NetCDF Core version 1.0

Conformance classes used:
•       core

OGC 10-092, NetCDF Binary Encoding Extension Standard: NetCDF Classic and 64-bit Offset Format  version 1.0

Conformance classes used:
•       netCDF classic
•       netCDF 64-bit

OGC 11-165r2, CF-netCDF Data Model extension specification, version 2.0.1

Conformance classes used:
•       CF-netCDF  core
•       CF-netCDF Discrete Sampling
•       CF-netCDF Mapping onto ISO 19123

OGC 07-036, Geography Markup Language (GML) Encoding Standard, version 3.2.1

Conformance classes used:
•      GML writing

OGC 09-146r1, OGC^® Implementation Schema for Coverages, version 1.0

Conformance classes used:
•       gml-coverage

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 Coverage

Feature that acts as a function to return values from its range for any direct position within its spatiotemporal domain [OGC 07-111]

4.2 Data Model

A description of the organization of data in a manner that reflects an information structure [ISO/IEC 11179-1 Specification and standardization of data elements – Part 1: Framework]

NOTE: netCDF literature reports the following definition for data model: a way of thinking about scientific data by applying a data model theory. It is an abstraction that describes how datasets are represented and used. In computer terms, a data model can be thought of as equivalent to an abstract object model in Object Oriented Programming in that an abstract data model describes data objects and what methods can be used on them.

4.3 Feature

abstraction of real world phenomena [5].

NOTE A feature may occur as a type or an instance.

4.4 GMLCOV

Note that OGC Coverage Implementation Schema 1.1 supersedes its predecessor version, GML 3.2.1 Application Schema – Coverages version 1.0.1, nicknamed GMLCOV, in a backwards compatible way. Instead of naming the new version GMLCOV 1.1, the standard has been renamed to Coverage Implementation Schema (CIS) 1.1 to remedy misunderstandings caused by the initial title, such as that only a GML encoding is defined here (whereas in fact a format-independent implementable coverage model is established).

For simplicity, the GMLCOV shorthand is used throughout this specification.

4.5 Grid

Network composed of two or more sets of curves in which the members of each set intersect the members of the other sets in a algorithmic way [5]

NOTE: the curves partition a space into grid cells.

4.6 MIME

MIME (Multi-Purpose Internet Mail Extensions) is an extension of the original Internet e-mail protocol that lets people use the protocol to exchange different kinds of data files on the Internet: audio, video, images, application programs, and other kinds, as well as the ASCII text handled in the original protocol, the Simple Mail Transfer Protocol (SMTP).

4.7 Multipart messages

Through the use of the multipart type, MIME allows mail messages to have parts arranged in a tree structure where the leaf nodes are any non-multipart content type and the non-leaf nodes are any of a variety of multipart types.

4.8 Multi-point coverage

A discrete coverage which is characterized by a finite domain consisting of points. Generally, the domain is a set of irregularly distributed points.

A set of hydrographic soundings is an example of a discrete point coverage.

4.9 NetCDF

NetCDF is a standard for data on complex grids –curvilinear in XY; sigma and density-related in Z; climatological and artificial calendars in T; and heading towards “tile mosaics” and 5D forecast ensembles in the near future.

4.10 Web address

For purposes of this document, Web address is an HTTP URL that allows applications to access remote (optionally, time-aggregated collections of) netCDF-CF files (virtual datasets – often terabyte sized) through netCDF API –as if they were local netCDF files.

5          Conventions

5.1           Namespace prefix conventions

The following namespaces are used in this document. The prefix abbreviations used constitute conventions used here, but are notnormative. The namespaces to which the prefixes refer are normative, however.

5.2           CF-netCDF data model

This document does not contain inline a formal specification of the CF-netCDF data model. In fact, it is available as an OGC specification [OGC 11-165r2]. This specification formalizes the CF-netCDF data model mapping onto the ISO 19123 coverage geometry model.

For reader’s convenience, the CF-netCDF data model is shown in Figure 1, Figure 2, and Figure 3. An example of CF-netCDF dataset is reported in the Annex B.