This document defines an extension to WCS2.1, namely the extraction of data contained within a polygon defined either by a set of points or the radius and position of a circle point consisting of an information model and an XML encoding for the following two operations:
GetCapabilities — a WCS function that describes the services and operations via a GetCapabilities document.
GetPolygon — a WCS function that supports this operation to extract data from a multidimensional cube that lie within a polygon.
Metadata and vocabularies are defined that provide interoperability of these operations and documents using common semantics. The information model proposed supports MetOcean specific concepts and its user community, but these constructs may be useful and applicable to other communities.
The following are keywords to be used by search engines and document catalogues.
ogcdoc, OGC document, WCS, coverage, meteorology, oceanography, NWP, analysis, polygon observation, measurement, simulation, O&M and MetOcean
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. Security Considerations
No security considerations have been made for this standard.
V. Submitting Organizations
The following organizations submitted this Document to the Open Geospatial Consortium (OGC):
- Met Office, UK
- NOAA’s National Weather Service
All questions regarding this submission should be directed to the editor or the submitters:
|Peter Trevelyan||Met Office, UK|
National Oceanic and Atmospheric Administration (NOAA)
National Weather Service (NWS)
National Oceanic and Atmospheric Administration (NOAA)
National Weather Service (NWS)
OGC MetOcean Application profile for WCS2.1: Part 2 MetOcean GetPolygon Extension
The purpose of the GetPolygon operation is to extract data contained within a polygon defined either by a set of points or the radius and position of a circle point. The need for the GetPolygon operation stems from active members of the OGC MetOcean Domain Working Group (DWG) who saw a manifest need for extraction of such information from gridded datasets.
This work has been done by members of the OGC MetOcean Domain Working Group.
This standard defines:
An amended GetCapabilities operation response that will list the GetPolygon operation and specify the token in the Sections element of the GetCapabilities request.
A new operation “GetPolygon” that is used to extract data from a multidimensional cube contained within a polygon.
The conformance classes that describe the GetPolygon operation.
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[multiblock footnote omitted].
In order to conform to this OGC™ interface standard, a software implementation shall choose to implement:
Any one of the conformance levels specified in Annex A (normative).
All requirements-classes and conformance-classes described in this document are owned by the standard(s) identified.
Requirements and conformance test URIs defined in this document are relative to: http://www.opengis.net/spec/WCS_application-profile_metocean_polygon/1.0/
This document establishes the following requirements and conformance classes:-
GetPolygon of URI http://www.opengis.net/spec/WCS_application-profile_metocean_polygon/1.0/req/getPolygon getPolygon at a conceptual level in clause 8.1
The corresponding conformance class is getPolygon with URI http://www.opengis.net/spec/WCS_application-profile_metocean_polygon/1.0/conf/getPolygon See A.1
PolygonDescriptionRing of http://www.opengis.net/spec/WCS_application-profile_metocean_polygon/1.0/req/getPolygon/PolygonDescriptionRing defining the PolygonRing at a conceptual level in clause 8.2
The corresponding conformance class is PolygonDescriptionRing with URI http://www.opengis.net/spec/WCS_application-profile_metocean_polygon/1.0/conf/getPolygon/PolygonDescriptionRing. See A.2
PolygonDescriptionCircle of http://www.opengis.net/spec/WCS_application-profile_metocean_polygon/1.0/req/getPolygon/PolygonDescriptionCircle defining the PolygonCircle at a conceptual level in clause 8.3
The corresponding conformance class is PolygonDescription with URI http://www.opengis.net/spec/WCS_application-profile_metocean_polygon/1.0/conf/getPolygon/PolygonDescriptionCircle. See A.3
SubsetByTrim of http://www.opengis.net/spec/WCS_application-profile_metocean_polygon/1.0/req/getPolygon/SubsetByTrim defining the VerticalTemporalDescription at a conceptual level in clause 8.4
The corresponding conformance class is VerticalTemporalDescription with URI http://www.opengis.net/spec/WCS_application-profile_metocean_polygon/1.0/conf/getPolygon/SubsetByTrim. See A.4
SubsetByInterpolation of http://www.opengis.net/spec/WCS_application-profile_metocean_polygon/1.0/req/getPolygon/SubsetByInterpolation defining the VerticalTemporalDescription at a conceptual level in clause 8.5
The corresponding conformance class is VerticalTemporalDescription with URI http://www.opengis.net/spec/WCS_application-profile_metocean_polygon/1.0/conf/getPolygon/SubsetByInterpolation. See A.5
GetPolygon-post-xml of http://www.opengis.net/spec/WCS_application-profile_metocean/1.0/req/getPolygon-post-xml defining the VerticalTemporalDescription at a conceptual level in clause 8.6
The corresponding conformance class is VerticalTemporalDescription with URI http://www.opengis.net/spec/WCS_application-profile_metocean/1.0/conf/getPolygon-post-xml seeA.6
3. Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
Policy SWG: OGC 08-131r3, The Specification Model — Standard for Modular specifications. Open Geospatial Consortium (2009). https://portal.ogc.org/files/?artifact_id=34762&version=2
ISO: ISO/TS 19103:2005, Geographic information — Conceptual schema language. International Organization for Standardization, Geneva (2005). https://www.iso.org/standard/37800.html
ISO: ISO 8601:2004, Data elements and interchange formats — Information interchange — Representation of dates and times. International Organization for Standardization, Geneva (2004). https://www.iso.org/standard/40874.html
ISO: ISO 19107:2019, Geographic information — Spatial schema. International Organization for Standardization, Geneva (2019). https://www.iso.org/standard/66175.html
ISO: ISO 19111:2007, Geographic information — Spatial referencing by coordinates. International Organization for Standardization, Geneva (2007). https://www.iso.org/standard/41126.html
ISO: ISO 19123:2005, Geographic information — Schema for coverage geometry and functions. International Organization for Standardization, Geneva (2005). https://www.iso.org/standard/40121.html
ISO: ISO 19115-1:2014, Geographic information — Metadata — Part 1: Fundamentals. International Organization for Standardization, Geneva (2014). https://www.iso.org/standard/53798.html
ISO: ISO 19156:2011, Geographic information — Observations and measurements. International Organization for Standardization, Geneva (2011). https://www.iso.org/standard/32574.html
ISO: ISO 19136:2007, Geographic information — Geography Markup Language (GML). International Organization for Standardization, Geneva (2007). https://www.iso.org/standard/32554.html
Peter Baumann: OGC 17-089r1, OGC Web Coverage Service (WCS) 2.1 Interface Standard — Core. Open Geospatial Consortium (2018). http://docs.opengeospatial.org/is/17-089r1/17-089r1.html
Marie-Françoise Voidrot-Martinez, Chris Little, Jürgen Seib, Roy Ladner, Adrian Custer, Jeff de La B: OGC 12-111r1, OGC Best Practice for using Web Map Services (WMS) with Time-Dependent or Elevation-Dependent Data. Open Geospatial Consortium (2014). https://portal.ogc.org/files/?artifact_id=56394
Simon Cox: OGC 10-025r1, Observations and Measurements — XML Implementation. Open Geospatial Consortium (2011). https://portal.ogc.org/files/?artifact_id=41510
Alexandre Robin: OGC 08-094r1, OGC® SWE Common Data Model Encoding Standard. Open Geospatial Consortium (2011). https://portal.ogc.org/files/?artifact_id=41157
Peter Baumann, Eric Hirschorn, Joan Masó: OGC 09-146r8, OGC Coverage Implementation Schema with Corrigendum. Open Geospatial Consortium (2019). http://docs.opengeospatial.org/is/09-146r8/09-146r8.html
Arliss Whiteside Jim Greenwood : OGC 06-121r9, OGC Web Service Common Implementation Specification. Open Geospatial Consortium (2010). https://portal.ogc.org/files/?artifact_id=38867
UCUM: Unified Code for Units of Measure (UCUM) – Version 1.9, 2013, http://unitsofmeasure.org/
OMG UML 2.5.1, Unified Modeling Language. (2017). https://www.omg.org/spec/UML/2.5.1/
W3C: Extensible Mark-up Language (XML) – Version 1.0 (Fifth Edition), August 2008
W3C: XML Schema – Version 1.0 (Second Edition), October 2004
Peter Baumann, Jinsongdi Yu: OGC 12-039, OGC® Web Coverage Service Interface Standard — Scaling Extension. Open Geospatial Consortium (2014). https://portal.ogc.org/files/?artifact_id=54504
Peter Baumann, Jinsongdi Yu: OGC 12-049, OGC® Web Coverage Service Interface Standard — Interpolation Extension. Open Geospatial Consortium (2014). https://portal.ogc.org/files/?artifact_id=54502
Peter Baumann, Jinsongdi Yu: OGC 11-053r1, OGC® Web Coverage Service Interface Standard — CRS Extension. Open Geospatial Consortium (2014). https://portal.ogc.org/files/?artifact_id=54209
Peter Trevelyan, Paul Hershberg, Steve Olson: OGC 15-045r7, OGC MetOcean Application profile for WCS2.1: Part 0 MetOcean Metadata. Open Geospatial Consortium (2020)
4. Terms and definitions
For the purposes of this document, the following terms and definitions apply.
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.
This document also uses terms defined in the OGC Standard for Modular specifications (OGC 08-131r3), also known as the ‘ModSpec’. The definitions of terms such as standard, specification, requirement, and conformance test are provided in the ModSpec.
For the purposes of this document, the following additional terms and definitions apply. There is some variation in the specific use of some technical terms within the meteorological domain. We have attempted to follow common usage, referring where possible to the WMO No.306 http://www.wmo.int/pages/prog/www/WMOCodes.
numerical weather prediction model
A numerical weather prediction model is a mathematical model of the atmosphere and oceans used to predict the weather based on current weather conditions and are normally run at set times each day.
Synonyms for numerical weather prediction model: forecast model, NWP Model, simulation
An Example of a numerical weather prediction model: The ECMWF model that runs twice per day and creates a ten day prediction of the global atmosphere.
A typical numerical model simulation has the property of start and end time of the simulation. Intermediate times within the simulation are times for which the data may be used as estimate of the forecast conditions at that time. The use of time within this document does assume that the “Time Dimension” of the simulation falls within the start and end times of the simulation. The standard ISO 8601 notation is used to describe time.
Web Coverage Service 2.1 (WCS2.1)
Web Coverage Service (WCS) is an OGC standard that refers to the exchange of geospatial information as ‘coverages’: digital geospatial information representing space-varying phenomena.
The getCapabilities is a WCS operation involving a machine to machine communication. A getCapabilities request to a WCS server returns a list of what operations and services (“capabilities”) are being offered by that server.
A DescribeCoverage is a WCS operation involving a machine-to-machine communication. A DescribeCoverage request to a WCS server returns additional information about a coverage that a client wants to query. Generally speaking, a DescribeCoverage response includes information about the CRS, the metadata, the domain, the range and the formats available. A client generally will need to issue a DescribeCoverage request before it can make the proper GetCoverage request.
A Polygon, in this document, consist of a plane 2D shape with a depth and time dimension. The polygon may be multi-dimensional, and in the case of aviation is often four dimensions, i.e. x, y, z, t.
GetPolygon is a newly proposed MetOcean operation involving a machine to machine communication. A GetPolygon request to a WCS server returns a polygon coverage based on a trajectory path with a lateral and vertical extent (the polygon).
This sections provides details and examples for any conventions used in the document. Examples of conventions are symbols, abbreviations, use of XML schema, or special notes regarding how to read the document.
The normative provisions in this standard are denoted by the URI
All requirements and conformance tests that appear in this document are denoted by partial URIs which are relative to this base.
5.2. Abbreviated terms
GML Geography Mark-up Language
O&M Observations and Measurements
OGC Open Geospatial Consortium
NWP Numerical Weather Prediction
SWE OGC Sensor Web Enablement
UML Unified Modelling Language
WCS2.1 OGC Web Coverage Service version 2.1
WMO World Meteorological Organisation
XML W3C Extensible Markup Language
XSD W3C XML Schema Definition Language
5.3. Schema language
The XML implementation specified in this Standard is described using the XML Schema language (XSD) [XML Schema Part 1: Structures, XML Schema Part 2: Datatypes] and Schematron [ISO/IEC 19757-3, Information technology — Document Schema Definition Languages (DSDL) — Part 3: Rule-based validation — Schematron].
5.4. UML notation
The diagrams that appear in this standard are presented using the Unified Modeling Language (UML) static structure diagram.
Note: Within the context of this standard, the following color scheme is used to identify the package in which the class exists. This is just for informative purposes.
Blue: WCS2.1 plus extensions (rsub, scal, int and crs)
Orange: CIS (Coverage Implantation Schema 1.1)
Green: This standard
This standard defines a number of properties that require the use of codes or vocabulary items. In some cases, a list of terms are provided. The MetOcean Profile, on which this builds, has a specific vocabulary provided by the WMO (World Meteorological Office). These vocabularies are concerned with the naming of parameters (variables) used in the rangeSubset element, the coordinate reference systems (aka fixedSurfacetypeAndUnits) used in the srsName attribute, the units of measure, and the significance of time codes. The following table lists the references used within this document.
Table 1 — Summary of vocabularies within this standard
|Code list||Code reference|
|GRIB edition 2||http://codes.wmo.int/_grib2|
|Fixed surface types and units||http://codes.wmo.int/grib2/codeflag/_4.5|
7. Non-Normative (Informative) Material
The GetPolygon extension for WCS2.1 is an initiative of the MetOcean DWG to enhance the WCS2.1 core profile to extract coverages other than those extracted using the simple SLICE and TRIM methods provided by the core GetCoverage operation shown in Figure 1. For reference to the core WCS2.1 see OGC® Web Coverage Service 2.1 Interface Standard — Core (OGC 17-089r1). This specific extension is designed specifically to extract polygons from multidimensional cubes such as those created by numerical simulations (i.e. NWP) commonly found in the MetOcean community.
Figure 1 — WCS GetCoverage operation UML class diagram
The need for this work arises out of the growing need to transfer increasing amounts of data across networks. This can, and should, be done more efficiently by sub-setting the data. This profile specifies how data contained within a polygon may be extracted on the WCS server and transferred to the client. The document also details how the OGC’s WCS2.1 Core is extended to include an additional GetPolygon operation. Finally, the Coverage Implementation Schema (CIS1.1) is used to describe the vertical and temporal definition of the Polygon.
The WCS2.1 files (see https://portal.opengeospatial.org/files/?artifact_id=67116&version=1) that form the core standard and the extensions (see below) describe the GetCapabilities, DescribeCoverage and GetCoverage operations. The GetPolygon extension will use the following extensions to WCS core:
WCS Range Subsetting Extension, version 1.0.0, OGC 12-040
WCS Scaling Extension, version 1.0.0, OGC 12-039
WCS Interpolation Extension, version 1.0.0, OGC 12-049
WCS CRS Extension, version 1.0, OGC 11-053r1
The main benefit of WCS2.1 core to the MetOcean Profile, and specifically the getPolygon operation, is that it allows the description of a CIS1.1 Coverage (see Figure 2). This is important as CIS1.1 supports multi-dimensional coverages and the encoding of coverage types such as polygons.
Figure 2 — UML Diagram representing the coverage model (CIS 1.1)
7.2. A Short NWP (Numerical Weather Prediction) Primer
The term “NWP model” refers to a computer simulation used to forecast the future state of the ocean/atmosphere. A NWP model is normally “run” at a set time and repeated at regular intervals during the day. Each model run has a “Time Dimension” that has “start time”, “end time” and intermediate times. These intermediate times are at set intervals and often referred to as “forecast times”. For each “forecast time”, there will be an estimate of the atmospheric/oceanic conditions for that time. The model start time is a notional time that sets a “base” time for the time intervals. Thus a time interval may have an absolute time e.g. 2017-05-15T00:00:00Z. Or, a time interval may use the start time as reference e.g. PT30H. This signifies a time that is 30 hours ahead of the “start time”.
A “coverage” contains a “DomainSet” component describing the coverage’s domain (i.e. the locations for which values are stored in the coverage) and a “rangeSet” component containing the values of the coverage. A “coverage” also contains a RangeType element that describes the coverage’s range set data structure that consists of one or more fields (also referred to as parameters) that uses the SWE Common [OGC 08-094] DataRecord. The metadata component represents an extensible slot for metadata. The CIS1.1 UML diagram is shown in Figure 2.
7.3.1. 4D Coverages
A typical NWP forecast is normally a set of 2D rectified grids; although more advanced, grids may be used. A typical model run contains literally thousands of 2D grids and each may described using the CIS1.1 “General Grid” coverage model.
These coverages may described and accessed by using the OGC’s Web Coverage Service (WCS). The main services are “GetCapabilities”, “DescribeCoverage” and “GetCoverage”. A problem with this approach is that the metadata returned by the GetCapabilites response and the number of GetCoverage requests quickly becomes unmanageable. By creating a multidimensional coverage from the 2D coverages, the amount of data and number of GetCoverage requests are greatly reduced, often by a factor of 100.
The key to creating a multidimensional coverage is OGC’s CIS1.1 “Coverage” model. This model makes it much easier to describe the “Domain Set” as a multi-dimensional geometry object.
A typical numerical simulation has a number of different vertical coordinates (i.e. pressure, height above mean sea level, height above ground, surface, and max wind level). By forming a 4D coverage from all of the 2D coverages that share the same horizontal, vertical, and temporal domains, there is a significant reduction in number of coverages, thus reducing complexity. This is a challenge, as the vertical and temporal axes are not regular and need to be enumerated. The “GeneralGridCoverage” as described in CIS1.1 makes this possible.
This key concept therefore changes the traditional view of data as being a set of 2D fields; each with a level, level type, parameter name, and forecast period. We can now describe the whole atmosphere as a multidimensional cube with properties, e.g. temperature, wind speed, and humidity. This results in the ability to make multidimensional (4/5/* D) geospatial queries that are much more efficient, for both the WCS2.1 GetCoverage, GetPolygon and GetCorridor operations. It is important to note that there are special cases where the vertical axis has no vertical dependency (e.g. surface, max wind level). It is also possible that some parameters (e.g. temperature) may belong to more than one coverage (e.g. surface, isobaric, etc.)
A typical polygonal extraction pattern is show in Figure 3 using a set of points to describe the polygon. A special case of a polygon extraction is to use a circle with a radius and centre point position.