[SIS-357] EPSG factory should be able to create geocentric CRS as derived CRS

• Type: Improvement
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Affects Version/s: 0.7, 0.8, 1.0, 1.1, 1.2, 1.3, 1.4
• Fix Version/s: 1.5
• Component/s: Referencing
• Assignee: Martin Desruisseaux
• Reporter: Martin Desruisseaux
• Created: 05/May/17 10:00
• Updated: 23/Sep/23 10:30

Description

The EPSGDataAccess.createCoordinateReferenceSystem(String) queries the EPSG "Coordinate Reference System" table with more than 10 columns. Some relevant columns for this tasks are:

• COORD_REF_SYS_KIND (may be "geographic 2D", "geographic 3D", "geocentric", "projected", etc.)
• SOURCE_GEOGCRS_CODE (EPSG code of a GeographicCRS)
• PROJECTION_CONV_CODE (EPSG code of a CoordinateOperation)

Current EPSGDataAccess implementation uses the SOURCE_GEOGCRS_CODE and PROJECTION_CONV_CODE columns only if the COORD_REF_SYS_KIND is "projected". But the EPSG database sometime uses those columns for other kind of CRS. For example with WGS 84 coordinate reference systems:

• EPSG:4326 (geographic 2D) is derived from EPSG:4979 (geographic 3D) with conversion EPSG:15593 (geographic3D to geographic2D).
• EPSG::4979 (geographic 3D) is derived from EPSG::4978 (geocentric) with conversion EPSG:15592 (geocentric to geographic3D).

We could modify EPSGDataAccess as below:

• Unconditionally build a coordinate operation from SOURCE_GEOGCRS_CODE and PROJECTION_CONV_CODE columns instead than only in the projected CRS case.
• Above operation is mandatory for projected CRS case, optional for all other.
• If an operation was found, use it for creating a derived CRS like below:

CoordinateOperation op = …,
CoordinateReferenceSystem base = ...;
if (op instanceof Conversion && base instanceof SingleCRS) {
    crs = DefaultDerivedCRS.create(properties, (SingleCRS) base, (Conversion) op, cs);
}

However we may want to keep Well Known Text (WKT) outputs simple by formatting the CRS as if it was a non-derived CRS. How to handle that is an open question.