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Provide an ElevationGridCoverage encoding example and guidelines for identifying the Vertical CRS

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Description

Handling of Vertical CRS is specified at an abstract level in Section 6.2.1.4.1 - “General mechanism for the identification of the vertical CRS” of TG on EL, more concretely under “Vertical CRS linkage to Elevation grid coverages” heading and Figure 23 (on the basis of ISO 19123). However, this is not accompanied of clear rules on how to identify the vertical CRS at implementation level (i.e. in GMLCOV schema).

From recent discussion under the OGC, there seem to be some consensus on handling the Vertical CRS description in the ‘rangeType’ component of GMLCOV ‘RectifiedGridCoverage’, specifically under the ‘referenceFrame’ attribute of a ‘swe:Quantity’ element.

Additionally, the example provided in Section 9.4.1.2 of TG on EL is taken from Section 6.7 of OGC 09-146r2 GML Application Schema, and it is addressing Radiance and not Elevation. Therefore it is not very useful in the TG on EL.

The following changes are proposed to clarify the mentioned aspects.

Discussion thread

https://themes.jrc.ec.europa.eu/discussion/view/42326/need-more-guidance-for-elevation-encoding-and-correct-example-for-elevationgridcoverage-on-the-basis-of-gmlcov-schema

Proposals for MIG-T

1) Add the following guideline at the end of Section 5.5.1.2.4 – “Attribute ElevationGridCoverage::rangeType”:

  • Quantity::referenceFrame attribute

    Identification of the vertical CRS used for referring the elevation values, which shall be always specified.

2) In Section 6.2.1.4.1 of TG on EL:

  • Add the following NOTE at the end of the text under heading “Vertical CRS linkage to Elevation 2.5D vector data and TIN models”:

"NOTE: The Coordinate Reference System association (‘CRS’ role) is implemented in GML by the 'srsName' property from geometric objects types - From OGC 07-036 GML, D.2.3.2 Geometry root (page 277):

'GM_Object' is represented by the 'AbstractGeometry' object element, the 'CRS' role is represented by the 'srsName' property.”"

  • Add the following NOTE at the end of the text under heading “Vertical CRS linkage to Elevation 2D vector data with the elevation property attribute”:

    "NOTE: The Coordinate Reference System association (‘CRS’ role) is implemented in GML by the 'srsName' property from the coordinate list - From OGC 07-036 GML, D.2.3.4 Coordinate geometry (page 283):

    'DirectPosition' is represented in GML as a type with simple content where the 'coordinate' attribute is mapped to a list of doubles. The 'coordinateReferenceSystem' role is represented by a 'srsName' attribute property and 'dimension' is represented by an optional attribute property of the same name (type is positiveInteger).”"

     

  • Add the following NOTE at the end of the text under heading “Vertical CRS linkage to Elevation grid coverages”:

"NOTE: According Requirement 13 of OGC 09-146r2 GML Application Schema Coverages (GMLCOV), “A coverage of type ‘RectifiedGridCoverage’ shall have a domain that is a ‘gml:RectifiedGrid’ geometry”, and this geometry is restricted to 2 dimensions in this specification.

Hence, the Coordinate Reference System association (‘CRS’ role) is implemented in GML / GMLCOV by:

  • For specifying the horizontal CRS: the 'srsName' property from ‘gml:RectifiedGrid’.

  • For specifying the vertical CRS to which the elevation values in the ‘rangeSet’ of the coverage are referenced: the ‘referenceFrame’ property from the ‘swe:Quantity’ element (defined in OGC 08-094r1 SWE Common Data Model), as part of the 'rangeType' component of the coverage - as already stated in Section 5.5.1.2.4 of this specification."

3) Section 9.4.1.2 – “Default encoding(s) for application schema ElevationGridCoverage (coverage data)” of TG on EL:

  • Replace the GMLCOV example currently provided in this section with the following example prepared in collaboration of Thematic Cluster #3:

Example Elevation grid coverages - single coverage (tested) - FINAL

Edit the full description of the example, replacing the text:

“EXAMPLE The following is a complete RectifiedGridCoverage instance (taken from [OGC 09-146r2]), using the base type RectifiedGridCoverage defined in the OGC GML Application Schema – Coverages available from http://schemas.opengis.net/gmlcov/1.0/.”

With the following text proposal:

“EXAMPLE The following is a complete RectifiedGridCoverage instance showing an ElevationGridCoverage using GML multipart representation.”

Note that this example shall be placed at the same level of Section 9.4.1.2 or at the level of the new subsection on “GML multipart representation” (newly proposed here - https://themes.jrc.ec.europa.eu/pages/view/54815/clarify-structure-of-coverage-encoding-related-sections-in-tgs-default-encodings-and-alternative-encodings).

Comments

Elevation, Orthoimagery, Reference Systems and Geographical Grids Cluster

Elevation, Orthoimagery, Reference Systems and Geographical Grids Cluster

INSPIRE Thematic Cluster Elevation, Orthoimagery, Reference systems, Geographical grids - Join this group to share your knowkledge, learn and collaborate in solving issues related to the Elevation, Orthoimagery, Reference systems and Geographical grids themes

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