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.)

7.4.1.  Encoding

Encoding of the polygon in GML is likely to be very verbose and in such cases, a binary format should be used. An alternative to GML e.g. JSON, would however provide a very accessible encoding.

8.1.1.  Requirements class overview

The GetPolygon requirements class defines the structure of the GetPolygon operation.

8.1.2.  Metoceanpolygon::GetPolygon

The new operation GetPolygon allows for the extraction of a polygon. The extra conformance classes are used to further define the possible options outlined here in the getPolygon conformance class. The GetPolygon operation is derived from wcs:GetCoverage and inherits the version and service elements. The GetPolygon properties are listed in Table 2.

Table 2 — METOCEANPOLYGON::GetPolygon properties

8.1.3.  rsub::RangeSubset

The RangeSubset properties are listed in Table 3.

Table 3 — RSUB::RangeSubset properties

8.1.4.  rsub::RangeSubset

The RangeSubset properties are listed in Table 4.

Table 4 — RSUB::RangeSubset properties

8.2.1.  Requirements class overview

This clause establishes the PolygonDescriptionRing requirements class. Clients and servers supporting the requirements class support the extraction of a polygon from a multidimensional data cube using the “PolygonRing” definition of a polygon must conform this requirements class.

Figure 5 — PolygonDescriptionRing UML class diagram

8.2.2.  Requirements class overview

The PolygonDescription requirements class defines the structure of the polygon as extracted by the GetPolygon operation. The Polygon may be defined in one of two ways; either be a ring or a circle. The two methods are described as “per Ring” or “per Circle”.

8.2.3.  PolygonDescription as per PolygonRing

In this clause the polygon will be defined as a set of points to form a ring. The PolygonDescription properties, as per PolygonRing, are listed in Table 5.

Table 5 — METOCEANPOLYGON::PolygonDescription properties as per PolygonRing

8.2.4.  PolygonRing Properties

Table 6 — Table 6 METOCEANPOLYGON::PolygonRing properties

8.2.5.  Polygon Properties

Table 7 — GML::Polygon properties

8.3.1.  Requirements class overview

This clause establishes the PolygonDescriptionCircle requirements class. Clients and servers supporting the requirements class support the extraction of a polygon from a multidimensional coverage using the “PolygonCircle” definition of a polygon, must conform this requirements class..

Figure 6 — PolygonDescription UML class diagram

8.3.2.  PolygonDescription as per PolygonCircle

In this clause the polygon will be defined as a circle with centre point and radius. The PolygonDescription properties, as per PolygonCircle, are listed in Table 8.

Table 8 — METOCEANPOLYGON::PolygonDescription properties as per PolygonCircle

8.3.3.  PolygonDescription as per PolygonCircle

Table 9 — GML::Polygon properties as per PolygonCircle

8.3.4.  GML::Polygon as per PolygonCircle

A Polygon is a special surface that is defined by a single surface . The boundary of this patch is coplanar and the polygon uses planar interpolation in its interior. The elements exterior and interior describe the surface boundary of the polygon. The Polygon properties, as per PolygonCircle, are listed in Table 10.

Table 10 — GML::Polygon properties as per PolygonCircle

8.3.5.  GML::Ring

A ring is used to represent a single connected component of a surface boundary as specified in ISO 19107:2003, 6.3.6. Every gml:curveMember references or contains one gml:Curve. In the context of a ring, a curve describes the boundary of the surface. The curve is described by referencing one or more gml:segments. The Ring properties are listed in Table 11.

Table 11 — GML::Ring

8.3.6.  GML::Curve

A curve is composed of one or more curve segments. The element gml:segments encapsulates the segments of the curve. The Curve properties are listed in Table 12 GML::Curve.

Table 12 — GML::Curve

8.3.7.  GML:: CircleByCenterPoint

A gml:CircleByCenterPoint is a gml:ArcByCenterPoint with identical start and end angle to form a full circle. This representation can be used only in 2D. The CircleByCenterPoint properties are listed in Table 13 GML::CircleByCenterPoint.

Table 13 — GML::CircleByCenterPoint

8.4.1.  Requirements class overview

The polygon extraction pattern uses a 2D polygon, that is replicated over selected levels in the vertical dimension and selected times in the temporal dimension. These levels and times are selected by “trimming” the MetOcean cube in the vertical and temporal dimensions.

8.4.2.  SubsetByTrim Properties

The limits define the upper and lower times and levels to be extracted from the parent MetOcean cube. The extraction, as per the polygon, is only extracted on levels and times from the MetOcean cube that lie between the upper and lower limits. The SubsetByTrim properties are listed in Table 14.

Table 14 — METOCEANPOLYGON::SubsetByTrim properties

8.4.3.  DimensionTrim Properties

The DimensionTrim properties are listed in Table 15.

Table 15 — METOCEANPOLYGON::DimensionTrim properties

8.4.4.  An example an encoding

<?xml version="1.0" encoding="UTF-8"?>
<metoceanpolygon:GetPolygon
    xmlns:wcs20="http://www.opengis.net/wcs/2.0"
    xmlns:rsub="http://www.opengis.net/wcs/range-subsetting/1.0"
    xmlns:metoceanpolygon="http://www.opengis.net/wcs/metoceanProfile_getPolygon/1.0"
    xmlns:cis="http://www.opengis.net/cis/1.1/gml"
    xmlns:gml="http://www.opengis.net/gml/3.2"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.opengis.net/wcs/metoceanProfile_getPolygon/1.0 http://schemas.opengis.net/wcs/metoceanProfile/1.0/wcsMetOceanGetPolygon.xsd"
service="WCS" version="2.0.0">
<wcs20:CoverageId>CoverageId0</wcs20:CoverageId>
<rsub:RangeSubset>
<rsub:RangeItem>
<rsub:RangeComponent>Temperature</rsub:RangeComponent>
</rsub:RangeItem>
<rsub:RangeItem>
<rsub:RangeComponent>Wind_Speed</rsub:RangeComponent>
</rsub:RangeItem>
<rsub:RangeItem>
<rsub:RangeComponent>Wind_Direction</rsub:RangeComponent>
</rsub:RangeItem>
</rsub:RangeSubset>
<metoceanpolygon:polygonDescription>
<metoceanpolygon:PolygonDescription>
<metoceanpolygon:polygonGeometry>
<metoceanpolygon:PolygonRing gml:id="ID000">
<gml:Polygon gml:id="Model_Boundary-Geometry"
uomLabels="deg deg" axisLabels="lat Lon"
srsDimension="2"
srsName="http://www.opengis.net/def/crs/EPSG/0/4326">
<gml:exterior>
<gml:LinearRing>
<gml:posList> -90.0 -180.0 90.0 -180.0 90.0 180.0
-90.0 180.0 -90.0 -180.0 </gml:posList>
</gml:LinearRing>
</gml:exterior>
</gml:Polygon>
</metoceanpolygon:PolygonRing>
</metoceanpolygon:polygonGeometry>
<metoceanpolygon:verticaTemporalDescription>
<metoceanpolygon:VerticalTemporalDescription>
<metoceanpolygon:subsetByTrim>
<metoceanpolygon:SubsetByTrim>
<metoceanpolygon:dimensionTrim>
<metoceanpolygon:DimensionTrim uomLabel="hPA">
<metoceanpolygon:dimension>Pressure</metoceanpolygon:dimension>
<metoceanpolygon:trimLow>200.0</metoceanpolygon:trimLow>
<metoceanpolygon:trimHigh>1000.0</metoceanpolygon:trimHigh>
</metoceanpolygon:DimensionTrim>
</metoceanpolygon:dimensionTrim>
<metoceanpolygon:dimensionTrim>
<metoceanpolygon:DimensionTrim uomLabel="ISO8601">
<metoceanpolygon:dimension>Time</metoceanpolygon:dimension>
<metoceanpolygon:trimLow>2017-05-14T00:00:00Z</metoceanpolygon:trimLow>
<metoceanpolygon:trimHigh>2017-05-15T00:00:00Z</metoceanpolygon:trimHigh>
</metoceanpolygon:DimensionTrim>
</metoceanpolygon:dimensionTrim>
</metoceanpolygon:SubsetByTrim>
</metoceanpolygon:subsetByTrim>
</metoceanpolygon:VerticalTemporalDescription>
</metoceanpolygon:verticaTemporalDescription>
</metoceanpolygon:PolygonDescription>
</metoceanpolygon:polygonDescription>
</metoceanpolygon:GetPolygon>

8.5.1.  Requirements class overview

This conformance class describes how a grid (target grid) is defined onto which the data contained within the polygon is extracted by interpolation. The “target” grid is only defined in the vertical and temporal dimensions as there is no interpolation in the horizontal. Thus the grid points will not be interpolated in the horizontal domain, but will be in the vertical and temporal dimensions.

8.5.2.  SubsetByInterpolation Properties

The SubsetByInterpolation properties are shown in Table 16.

Table 16 — METOCEANPOLYGON::SubsetByInterpolation properties

8.5.3.  Interpolation Properties

The Interpolation properties are listed in Table 17.

Table 17 — INT::Interpolation properties

8.5.4.  InterpolationPerAxis Properties

The InterpolationPerAxis properties are listed in Table 18.

Table 18 — INT::InterpolationPerAxis Properties

8.5.5.  CIS::GeneralGrid Properties

Describing the direct positions in multi-dimensional coordinate space, depending on the type of grid. The GeneralGrid properties are listed in Table 19.

Table 19 — CIS::GeneralGrid properties

8.5.6.  DisplacementAxisNest Properties

The CIS::DisplacementAxisNest combines several axes to a single “nest” where the coordinates are enumerated individually for each direct pos­ition. The properties are listed in Table 20.

Table 20 — CIS::DisplacementAxisNest properties

8.5.7.  GridLimits Properties

The grid limits in the CIS::Axis structure contains information about the grid boundaries in the coverage’s CRS. The GridLimits properties are listed in Table 21.

Table 21 — CIS::GridLimits properties

8.5.8.  Axis Properties

The Axis properties are listed in Table 22.

Table 22 — CIS::Axis properties

8.5.9.  IndexAxis Properties

The IndexAxis properties are listed in Table 23.

Table 23 — CIS::IndexAxis properties

8.5.10.  RegularAxis Properties

The RegularAxis properties are listed in Table 24.

Table 24 — CIS::RegularAxis properties

8.5.11.  IrregularAxis Properties

The IrregularAxis properties are listed in Table 25.

Table 25 — CIS::IrregularAxis properties

8.5.12.  An example an encoding

<?xml version="1.0" encoding="UTF-8"?>
<metoceanpolygon:GetPolygon xmlns:cis="http://www.opengis.net/cis/1.1/gml"
xmlns:gml="http://www.opengis.net/gml/3.2" xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:swe="http://www.opengis.net/swe/2.0" xmlns:ows="http://www.opengis.net/ows/2.0"
xmlns:gco="http://www.isotc211.org/2005/gco" xmlns:gmd="http://www.isotc211.org/2005/gmd"
xmlns:sml="http://www.opengis.net/sensorml/2.0" xmlns:gmlcov="http://www.opengis.net/gmlcov/1.0"
xmlns:om="http://www.opengis.net/om/2.0" xmlns:sam="http://www.opengis.net/sampling/2.0"
xmlns:sams="http://www.opengis.net/samplingSpatial/2.0"
xmlns:gts="http://www.isotc211.org/2005/gts"
xmlns:rsub="http://www.opengis.net/wcs/range-subsetting/1.0"
xmlns:gsr="http://www.isotc211.org/2005/gsr" xmlns:ns0="http://www.opengis.net/wcs/2.0"
xmlns:gss="http://www.isotc211.org/2005/gss"
xmlns:metocean="http://www.opengis.net/wcs/metoceanProfile/1.0"
xmlns:metoceanpolygon="http://www.opengis.net/wcs/metoceanProfile_getPolygon/1.0"
xmlns:wcs="http://www.opengis.net/wcs/2.1" xmlns:wcs20="http://www.opengis.net/wcs/2.0" xmlns:gmlrgrid="http://www.opengis.net/gml/3.3/rgrid"
xmlns:int="http://www.opengis.net/WCS_service-extension_interpolation/1.0"
xmlns:scal="http://www.opengis.net/WCS_service-extension_scaling/1.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.opengis.net/wcs/metoceanProfile_getPolygon/1.0 http://schemas.opengis.net/wcs/metoceanProfile/1.0/wcsMetOceanGetPolygon.xsd"
                            service="WCS" version="2.0.0">
<wcs20:CoverageId>CoverageId0</wcs20:CoverageId>
<rsub:RangeSubset>
<rsub:RangeItem>
<rsub:RangeComponent>Temperature</rsub:RangeComponent>
</rsub:RangeItem>
<rsub:RangeItem>
<rsub:RangeComponent>Wind_Speed</rsub:RangeComponent>
</rsub:RangeItem>
<rsub:RangeItem>
<rsub:RangeComponent>Wind_Direction</rsub:RangeComponent>
</rsub:RangeItem>
</rsub:RangeSubset>
<metoceanpolygon:polygonDescription>
<metoceanpolygon:PolygonDescription>
<metoceanpolygon:polygonGeometry>
<metoceanpolygon:PolygonCircle gml:id="ID000">
<gml:Polygon gml:id="ID001">
<gml:exterior>
<gml:Ring>
<gml:curveMember>
<gml:Curve gml:id="curve01">
<gml:segments>
<gml:CircleByCenterPoint numArc="1">
<gml:pos>27.10 -73.10</gml:pos>
<gml:radius uom="[nmi_i]">250</gml:radius>
</gml:CircleByCenterPoint>
</gml:segments>
</gml:Curve>
</gml:curveMember>
</gml:Ring>
</gml:exterior>
</gml:Polygon>
</metoceanpolygon:PolygonCircle>
</metoceanpolygon:polygonGeometry>
<metoceanpolygon:verticaTemporalDescription>
<metoceanpolygon:VerticalTemporalDescription>
<metoceanpolygon:subsetByInterpolation>
<metoceanpolygon:SubsetByInterpolation>
<metoceanpolygon:interpolation>
<int:Interpolation>
<int:globalInterpolation>http://www.opengis.net/def/interpolation/OGC/1/linear</int:globalInterpolation>
<int:InterpolationPerAxis>
<int:axis>Time</int:axis>
<int:interpolationMethod>http://www.opengis.net/def/interpolation/OGC/1/linear</int:interpolationMethod>
</int:InterpolationPerAxis>
<int:InterpolationPerAxis>
<int:axis>pressure</int:axis>
<int:interpolationMethod>http://www.opengis.net/def/interpolation/OGC/1/barycentric</int:interpolationMethod>
</int:InterpolationPerAxis>
</int:Interpolation>
</metoceanpolygon:interpolation>

<metoceanpolygon:generalGrid>
<cis:GeneralGrid srsName="http://www.opengis.net/def/crs-compound?
1=http://http://www.opengis.net/def/crs/OGC/0/Time&amp;
2=http://www.codes.wmo.int/GRIB2/table4.5/IsobaricSurface"
axisLabels="Time pressure">
<cis:IrregularAxis uomLabel="ISO8601" axisLabel="Time">
<cis:C>PT0H</cis:C>
<cis:C>PT6H</cis:C>
<cis:C>PT12H</cis:C>
<cis:C>PT18H</cis:C>
<cis:C>PT24H</cis:C>
<cis:C>PT30H</cis:C>
<cis:C>PT36H</cis:C>
<cis:C>PT42H</cis:C>
<cis:C>PT48H</cis:C>
</cis:IrregularAxis>

<cis:IrregularAxis uomLabel="hPa" axisLabel="pressure">
<cis:C>1000.0</cis:C>
<cis:C>950.0</cis:C>
<cis:C>850.0</cis:C>
<cis:C>500.0</cis:C>
<cis:C>300.0</cis:C>
<cis:C>250.0</cis:C>
<cis:C>200.0</cis:C>
</cis:IrregularAxis>

<cis:GridLimits srsName="http://www.codes.wmo.int/def/crs/OGC/0/Index4D" axisLabels="k l" >
<cis:IndexAxis axisLabel="k" lowerBound="0" upperBound="8"/>
<cis:IndexAxis axisLabel="l" lowerBound="0" upperBound="6"/>
</cis:GridLimits>
</cis:GeneralGrid>
</metoceanpolygon:generalGrid>
</metoceanpolygon:SubsetByInterpolation>
</metoceanpolygon:subsetByInterpolation>
</metoceanpolygon:VerticalTemporalDescription>
</metoceanpolygon:verticaTemporalDescription>
</metoceanpolygon:PolygonDescription>
</metoceanpolygon:polygonDescription>
</metoceanpolygon:GetPolygon>