C++ API: add methods to build Geographic/Vertical Offset conversions

[rouault]

Cf #3411 (comment)

@hernando Please report if it works. I've just compiled tested this

[hernando]

Yes, this works.

[hernando]

I have a question though. I tried to create a ProjectedCRS of a DerivedGeographicCRS to use the same technique for the vertical offset but even if the construction didn't fail, the deriving conversion was stripped.
I tried to pass this WKT2 string to projinfo and it didn't like it:

PROJCRS["CH1903+ / LV95 with height offset",
    BASEGEOGCRS["CH1903+ with height offset",
        BASEGEOGCRS["CH1903+",
            DATUM["CH1903+",
                ELLIPSOID["Bessel 1841",6377397.155,299.1528128,
                    LENGTHUNIT["metre",1]]],
            PRIMEM["Greenwich",0,
                ANGLEUNIT["degree",0.0174532925199433]]],
        DERIVINGCONVERSION["Ellipsoidal to gravity related height",
            METHOD["Geographic3D offsets",
                ID["EPSG",9660]],
            PARAMETER["Latitude offset",0,
                ANGLEUNIT["degree",0.0174532925199433],
                ID["EPSG",8601]],
            PARAMETER["Longitude offset",0,
                ANGLEUNIT["degree",0.0174532925199433],
                ID["EPSG",8602]],
            PARAMETER["Vertical Offset",0,
                LENGTHUNIT["metre",1],
                ID["EPSG",8603]]],
        CS[ellipsoidal,3],
            AXIS["geodetic latitude (Lat)",north,
                ORDER[1],
                ANGLEUNIT["degree",0.0174532925199433,
                    ID["EPSG",9122]]],
            AXIS["geodetic longitude (Lon)",east,
                ORDER[2],
                ANGLEUNIT["degree",0.0174532925199433,
                    ID["EPSG",9122]]],
            AXIS["custom height (H)",up,
                ORDER[3],
                LENGTHUNIT["metre",1,
                    ID["EPSG",9001]]]],
    CONVERSION["Swiss Oblique Mercator 1995",
        METHOD["Hotine Oblique Mercator (variant B)",
            ID["EPSG",9815]],
        PARAMETER["Latitude of projection centre",46.9524055555556,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8811]],
        PARAMETER["Longitude of projection centre",7.43958333333333,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8812]],
        PARAMETER["Azimuth of initial line",90,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8813]],
        PARAMETER["Angle from Rectified to Skew Grid",90,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8814]],
        PARAMETER["Scale factor on initial line",1,
            SCALEUNIT["unity",1],
            ID["EPSG",8815]],
        PARAMETER["Easting at projection centre",2600000,
            LENGTHUNIT["metre",1],
            ID["EPSG",8816]],
        PARAMETER["Northing at projection centre",1200000,
            LENGTHUNIT["metre",1],
            ID["EPSG",8817]]],
    CS[Cartesian,3],
        AXIS["(E)",east,
            ORDER[1],
            LENGTHUNIT["metre",1]],
        AXIS["(N)",north,
            ORDER[2],
            LENGTHUNIT["metre",1]],
        AXIS["custom height (H)",up,
            ORDER[3],
            LENGTHUNIT["metre",1]]]

Is this not allowed?

[hernando]

I have a similar problem if I try to created a derived geographic from a bound (although this one doesn't surprise me as much).

[rouault]

I tried to create a ProjectedCRS of a DerivedGeographicCRS to use the same technique for the vertical offset but even if the construction didn't fail, the deriving conversion was stripped.
I tried to pass this WKT2 string to projinfo and it didn't like it:

I don't see anything obvious in OGC Abstract Topic 2 spec (https://docs.opengeospatial.org/as/18-005r4/18-005r4.html ) that would be forbid this, but the WKT2 spec at http://docs.opengeospatial.org/is/18-010r7/18-010r7.html#74 clearly has no provision for this. You could ask https://lists.ogc.org/mailman/listinfo/coordtran.wg if it is intended or not.

[hernando]

I tried to subscribe to the mailing list, but I'm still waiting for a confirmation. In the meantime I tried something different that seemed to unveil a bug in PROJ. I will submit another issue with the details.

[rouault]

but I'm still waiting for a confirmation

check your spam folder

[hernando]

but I'm still waiting for a confirmation

check your spam folder

That was it, thanks!