New GeoTIFF model CRS for geocentric CRS (spherical 2D)

[desruisseaux]

Purpose

This pull request proposes the addition of a geodetic CRS for the spherical case. It reuses the GeoTIFF existing definition of datum (by comparison, the addition of engineering CRS in a separated pull request will require a new datum definition).

GeoTIFF does not distinguishes Coordinate System (CS) from Coordinate Reference System (CRS). The two concepts are defined together. GeoTIFF already supports geocentric coordinate reference system, but only in association with Cartesian coordinate system. There is no geographic CRS associated to spherical coordinate system.

Sections to edit

• Prime Meridian defines geodetic latitudes, then said that other types are out of scope. We may need to add the definition of spherical (or geocentric) latitude first.
• Requirements Class Units GeoKeys needs a new bullet point for GeogAngularUnitsGeoKey for the axes in user-defined geocentric spherical 2D CRSs. Note: the Geog part in the name is not quite right in this context, but a precedent exists with GeogLinearUnitsGeoKey applied to geocentric Cartesian CRS.
• Defining Model Coordinate Reference Systems needs a new type: "Geocentric CRS (Spherical 2D)". This two-dimensional CS has no radius axis.
• Requirements Class GTModelTypeGeoKey needs a new type: "Geocentric CRS (Spherical 2D)". This two-dimensional CS has no radius axis.
• User-defined Model Coordinate Reference Systems with the addition of the following sub-sections:

User-defined geocentric CRS (Spherical 2D)

For a user-defined geocentric CRS (spherical 2D) the user is expected to provide:

• geocentric coordinate reference system name (through the GeodeticCitationGeoKey);
• geodetic datum through the GeodeticDatumGeoKey, either:
  • the geodetic datum code (if available through standard EPSG code), or
  • user-defined geodetic datum name and other defining information:
    • the geodetic datum name (through the GeodeticCitationGeoKey),
    • the ellipsoid (through the EllipsoidGeoKey, see User-defined ellipsoid), and
    • the prime meridian (through the PrimeMeridianGeoKey, see User-defined prime meridian);
  • axis unit through the GeogAngularUnitsGeoKey, either:
    • angle unit code (if available through standard EPSG code), or
    • user-defined angle unit name (through the GeodeticCitationGeoKey) and scaling from SI base unit of radian (through the GeogAngularUnitSizeGeoKey); and
  • if the CRS uses a user-defined prime meridian, prime meridian Greenwich longitude unit through the GeogAngularUnitsGeoKey, either:
    • angle unit code (if available through standard EPSG code), or
    • user-defined angle unit name (through the GeodeticCitationGeoKey) and scaling from SI base unit of radian (through the GeogAngularUnitSizeGeoKey).

Open questions

The GeogAngularUnitsGeoKey name should be understood as GeodeticAngularUnitsGeoKey in this context. Should it be renamed? Note that a similar problem exists in current specification with GeogLinearUnitsGeoKey applied to geocentric Cartesian CRS.

References

• An Overview of Reference Frames and Coordinate Systems in the SPICE Context, in particular pages 24 and 25.