[PROJ] Describing custom vertical datums in wkt

[Eric Younkin - NOAA Federal]

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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