[SIS-317] On-the-fly Geographic3D ↔ CompoundCRS conversion when parsing/formatting WKT 1

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.6
• Fix Version/s: 0.7
• Component/s: Referencing
• Labels: WKT
• Assignee: Martin Desruisseaux
• Reporter: Martin Desruisseaux
• Created: 25/Feb/16 13:07
• Updated: 17/May/16 21:55
• Resolved: 25/Feb/16 18:25

Description

According the ISO 19111 standard, ellipsoidal heights can not be represented by standalone VerticalCRS instances. Instead they need to be part of a three-dimensional GeographicCRS instance. Version 2 of Well Known Text (WKT) format follows this specification, but version 1 had a different model. In WKT 1, three-dimensional GeographicCRS did not existed (it is not even possible to represent such CRS in WKT 1 because of heterogeneous units of measurement). Instead, WKT 1 used a CompoundCRS made of a two-dimensional GeographicCRS followed by a VerticalCRS.

Example (WKT 2 format):

GEODCRS[“WGS 84 (3D)”,
  DATUM[“World Geodetic System 1984”,
    ELLIPSOID[“WGS84”, 6378137.0, 298.257223563, LENGTHUNIT[“metre”, 1]]],
    PRIMEM[“Greenwich”, 0.0, ANGLEUNIT[“degree”, 0.017453292519943295]],
  CS[ellipsoidal, 3],
    AXIS[“Longitude (L)”, east, ORDER[1], ANGLEUNIT[“degree”, 0.017453292519943295]],
    AXIS[“Latitude (B)”, north, ORDER[2], ANGLEUNIT[“degree”, 0.017453292519943295]],
    AXIS[“Ellipsoidal height (h)”, up, ORDER[3], LENGTHUNIT[“metre”, 1]],
  AREA[“World”],
  BBOX[-90.00, -180.00, 90.00, 180.00]]

Same example than above, but using WKT 1 format:

COMPD_CS[“WGS 84 (3D)”,
  GEOGCS[“WGS 84”,
    DATUM[“World Geodetic System 1984”,
      SPHEROID[“WGS84”, 6378137.0, 298.257223563]],
      PRIMEM[“Greenwich”, 0.0],
    UNIT[“degree”, 0.017453292519943295],
    AXIS[“Longitude”, EAST],
    AXIS[“Latitude”, NORTH]],
  VERT_CS[“Ellipsoidal height”,
    VERT_DATUM[“Ellipsoid”, 2002],
    UNIT[“metre”, 1],
    AXIS[“Ellipsoidal height”, UP]]]

The WKT 1 structure is illegal in today's model, so we need to create it temporarily only during WKT 1 formatting and discard it immediately after. Conversely, the WKT parser needs to convert the above-cited CompoundCRS to a three-dimensional GeographicCRS on-the-fly.

Issue Links

• is depended upon by: SIS-303 Do not allow instantion of CompoudCRS with Geographic2D + ellipsoidal height