[PROJ] Needs a sanity check on Vertical Transformation

[Julien Schroder]

Thank you so much for your help it is much clearer now and a perfect timing
with the new release.
That is good to know about WGS84 and the resulting uncertainty, it is
slightly worrying, I will investigate that further.
Thank again

*Julien Schroder | Spatial Analyst | Photographer*
julien.schroder at gmail.com | +1 514 797 1464

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Le lun. 4 nov. 2019 à 07:36, Even Rouault <even.rouault at spatialys.com> a
écrit :

> Brian,
>
> I've been working on that very subject those last days. Pending work is in
> https://github.com/OSGeo/PROJ/pull/1710
> and
> https://github.com/OSGeo/proj-datumgrid/pull/60
>
> to add support for GEOID18 (as well as older GEOID versions for
> specialized
> purposes)
> For getting a value in the grids, PROJ will use the Datum of the
> GeographicCRS
> indicated in the EPSG database for the transformation record between
> NAVD88
> and the NAD83 realizations you mention below, possibly doing a horizontal
> adjustment to go back and forth between the initial and final CRS and this
> interpolation CRS. It will not go through WGS84.
>
> So for example (with the 2 above pull requests in place), transforming
> between
> NAD83(2011) + NAVD88 height and NA83(2011) (3D) shows that no horizontal
> adjustment is attempted:
>
> projinfo -s EPSG:6318+5703 -t EPSG:6319 --spatial-test intersects -o PROJ
> Candidate operations found: 2
> -------------------------------------
> Operation n°1:
>
> unknown id, Inverse of NAD83(2011) to NAVD88 height (3), 0.015 m, USA -
> CONUS
> - onshore
>
> PROJ string:
> +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert
> +xy_in=deg +xy_out=rad +step +proj=vgridshift +grids=g2018u0.gtx
> +multiplier=1
> +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap
> +order=2,1
>
> -------------------------------------
> Operation n°2:
>
> unknown id, Inverse of NAD83(2011) to NAVD88 height (2), 0.02 m, USA -
> Alaska
>
> PROJ string:
> +proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert
> +xy_in=deg +xy_out=rad +step +proj=vgridshift +grids=g2012ba0.gtx
> +multiplier=1 +step +proj=unitconvert +xy_in=rad +xy_out=deg +step
> +proj=axisswap +order=2,1
>
> Even
>
> > GEOID18 supersedes GEOID12B in CONUS and Puerto Rico.  GEOID12B should
> > be used in Alaska, Hawaii, Guam, and the Commonwealth of the Northern
> > Mariana Islands.  American Samoa should not be used for anything as
> > ASVD02 is no longer valid after earthquakes.
> >
> > A note on Geoids, Horizontal (Geometric) Datums and Vertical Datums.  To
> > get an orthometric height (aka elevation) in a particular vertical datum
> > you should use coordinates in a particular geometric datum with lat,lon,
> > and ellipsoid height along with a geoid model to get an orthometric
> > height.  For the US here are the appropriate datums and geoid models.
> > Note that the corresponding geoid models should be used based on the
> > datum of your coordinates. Also note the the original NAD 83 (1986) was
> > a 2D datum while all other realizations were 3D, aka geometric.
> >
> > Also note that if you are concerned about accuracy, WGS84 should be
> > avoided if at all possible.   WGS84 has a 3-5 meter uncertainty so any
> > time you use coordinates or use it as a pivot in transformations you
> > introduce this uncertainty into your coordinates.
> >
> > Table on geoids and NAD 83 realizations
> >
> > NAD 83 (realization)  Geoid Model     Regions
> > NAD 83 (1986)         N/A
> > NAD 83 (HARN)         GEOID96/GEOID99
> > NAD 83 (FBN)  GEOID03
> > NAD 83 (CORS96)       GEOID03
> > NAD 83 (NSRS2007)     GEOID09
> > NAD 83 (2011)         GEOID12B        HI, AK, Guam, CNMI
> > NAD 83 (2011)         GEOID18         CONUS, PRVI
> >
>
> --
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> http://www.spatialys.com
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