[pdal] NAD83 ellipsoid heights in entwine

[Kirk Waters - NOAA Federal]

This may also relate to a recent question on "Unknown" values in WKT
strings. I'm trying to create EPT files where my horizontal coordinates are
in NAD83 UTM and my vertical are ellipsoid heights. There isn't a lot of
information that I could find about how to set up the reference system for
something like that. The recommendations in the OGC GeoTIFF documentation
<http://docs.opengeospatial.org/is/19-008r4/19-008r4.html#section-D-3> suggest
you could do something like putting the projected 2D CRS in the horizontal
and the code/description for the geographic 3D reference system in the
vertical. There is no EPSG code for NAD83 ellipsoid heights except as part
of a 3D reference system.

Do you have any idea how I can specify those coordinates such that entwine
or pdal will understand it? They currently do not recognize the 3D CRS code
(e.g. 6319 for NAD83(2011) ellipsoid) as a legitimate vertical choice. Is
there a WKT for this that just isn't well known to me? I think it isn't
only NAD83 that has this issue. I have checked in with EPSG on this before.
The response didn't get me very far, but I'll put it here in case it helps
someone else.

In the ISO data model (which EPSG follows) ellipsoidal height has been
defined as the vertical component of a 3D geographic coordinate reference
system. An ellipsoidal height definitely cannot exist on its own as a 1D
vertical CRS, and therefore cannot be part of a compound 2D + 1D CRS..

However there has been some recent discussion regarding whether the concept
of a 3D projected CRS (easting northing with ellipsoidal height) is valid.
Superficially it sounds possible. But there are some in the geodetic parts
of NOAA and other national mapping agencies that are doubtful as to whether
the concept is valid. There are sound technical arguments for why the
concept may be invalid. These revolve around the map projection process
being tangential to the ellipsoid surface and not on it.

We in the IOGP geodesy subcommittee are seeking evidence of an appropriate
implementation. We have yet to have explained a valid case. Most of the
evidence we have collected so far turns out to be imagery being draped over
a TIN and it then transpires that the heights in the TIN are
gravity-related, not ellipsoidal. These are not academic questions, for if
a projected 3D CRS construct were to be possible, we can envisage a number
of ways in which the components could be created. For spatial data
management there are plusses and minuses in each of these. We wish to fully
understand the pros and cons before we contemplate populating what, if we
got it wrong, could turn out to be inappropriate records in the EPSG
Dataset.


While it seems like the EPSG answer says my reference system isn't valid,
I'm not convinced. Lidar collections are going to start in the 3D
coordinate system (lat, lon, ellipsoid heights). If they then have a geoid
applied to get to orthometric prior to projecting, that seems to be
considered valid (e.g. NAD83 UTM + NAVD88), but if you don't apply the
geoid, it's invalid. That makes no sense.

Thanks,

Kirk Waters, PhD, BJCP          | NOAA Office for Coastal Management
Applied Sciences Program      | 2234 South Hobson Ave
843-740-1227 (empty office)   | Charleston, SC 29405
843-324-2203 (cell during COVID)
coast.noaa.gov/digitalcoast
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