[PROJ] Describing custom vertical datums in wkt

[Even Rouault]

Eric,

Besides a few minor issues in the below WKT (use of single quotes, or 
non-ASCII double quote characters), more fundamental remarks:

- I don't think DerivedVerticalCRS is the appropriate modeling. 
http://docs.opengeospatial.org/is/18-010r7/18-010r7.html in §3.1.14 
mentions: "A derived coordinate reference system inherits its datum or 
reference frame from its base coordinate reference system.", so you 
can't have the derived CRS having VDATUM["NOAA Chart Datum"] and the 
base CRS VDATUM["NAD83(2011) Height"]. And if you look at the grammar at 
http://docs.opengeospatial.org/is/18-010r7/18-010r7.html#120, VDATUM[] 
is not allowed for the derived vertical CRS, and when PROJ re-exports to 
WKT2 this definition, it will omit it.

- One could question too if the derivation is a conversion. §14.2.1 
mentions "Because by definition coordinate conversions are exact, the 
attribute operation accuracy is not relevant and excluded from the 
deriving conversion WKT string". Using grids is probably a big unusual 
to define a conversion

- you can't use EPSG:6319 that is the Geographic 3D CRS as a (base) 
vertical CRS. Ellipsoidal heights in ISO-19111 can't exist as standalone 
vertical CRS. They must be associated with the horizontal coordinates.

- the proper model would probably be to define a NOAA Chart Datum 
vertical datum and vertical CRS, and define a transformation between 
EPSG:6319 and the NOAA Chart Datum vertical CRS. You could reference 
that transformation in a GEOIDMODEL[] node attached as a child of the 
VERTCRS[].

- besides importing/exporting/querying the attributes, PROJ will not be 
able currently to do anything useful (ie compute a PROJ pipeline between 
that and another vertical CRS) with a derived projected CRS.

Even

Le 26/03/2021 à 16:06, Eric Younkin - NOAA Federal via PROJ a écrit :
> Hello,
>
> As we move to registering our vdatum grids in EPSG, we needed to 
> develop a stop gap solution, describing our custom datums in WKT for 
> archival.  These would be derived datums based on elheight or geoid 
> height, with proj vgrid pipelines applied to get to water level 
> datum.  We are thinking about using something like this:
>
> VERTCRS["NOAA Chart Datum",
>                   BASEVERTCRS["NAD83(2011) Height",
> VDATUM["NAD83(2011) Height"],
> ID["EPSG",6319]],
>                   DERIVINGCONVERSION["NAD83(2011) Height to NOAA Mean 
> Lower Low Water",
>  METHOD["VDatum_VXXX gtx grid transformation",
> ID["EPSG",1084]],
> PARAMETERFILE['g2012bu0', 'core\\geoid12b\\g2012bu0.gtx',
>                                                   ID[“NOAA VDatum”, 
> “NAD83 to Geoid12B”, “10/23/2012”]],
> PARAMETERFILE['tss', 'CAORblan01_8301\\tss.gtx',
>                                                   ID[“NOAA VDatum”, 
> “Geoid12B to Tss", “06/20/2019”]],
> PARAMETERFILE['mllw', 'CAORblan01_8301\\mllw.gtx',
>                                                   ID[“NOAA VDatum”, 
> “Tss to Mean Lower Low Water”, “06/20/2019”]]],
>                     VDATUM["NOAA Chart Datum"],
>                     CS[vertical,1],
>                           AXIS["gravity-related height (H)",up],
>                           LENGTHUNIT["metre",1]]
>
> I know this isn't something we could use in PROJ in the same way that 
> a registered EPSG reference could, but we could at least have 
> something describing what we do for archival. I'm thinking of building 
> some code to manually translate a move between two of these WKT 
> strings into a pipeline operation.
>
> Hopefully that made sense.  Please let me know if you have any 
> suggestions.
>
> Thanks,
> Eric
>
> -- 
> Eric Younkin
> Physical Scientist
> NOAA OCS, Hydrographic Systems and Technology Branch
> 1315 East-West Highway
> N/CS11, Room 6604
> Silver Spring, MD 20910
> Office: 240-847-8208
> Cell: 828-331-8197
>
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