[PROJ] Problems with CGVD28 height

[Javier Jimenez Shaw]

Using the option "--bbox", if I have done correctly, I get this

PROJ_NETWORK=ON projinfo -s EPSG:4979 -t EPSG:32181+5713 -o proj
--spatial-test intersects --bbox '-53.6,47.7,-53.5,47.8'
Candidate operations found: 2
-------------------------------------
Operation No. 1:

unknown id, Inverse of Transformation from CGVD28 height to WGS 84
(ballpark vertical transformation, without ellipsoid height to vertical
height correction) + Inverse of NAD83 to WGS 84 (1) + MTM zone 1, unknown
accuracy, North America - onshore and offshore: Canada - Alberta; British
Columbia; Manitoba; New Brunswick; Newfoundland and Labrador; Northwest
Territories; Nova Scotia; Nunavut; Ontario; Prince Edward Island; Quebec;
Saskatchewan; Yukon. United States (USA) - Alabama; Alaska (mainland);
Arizona; Arkansas; California; Colorado; Connecticut; Delaware; Florida;
Georgia; Idaho; Illinois; Indiana; Iowa; Kansas; Kentucky; Louisiana;
Maine; Maryland; Massachusetts; Michigan; Minnesota; Mississippi; Missouri;
Montana; Nebraska; Nevada; New Hampshire; New Jersey; New Mexico; New York;
North Carolina; North Dakota; Ohio; Oklahoma; Oregon; Pennsylvania; Rhode
Island; South Carolina; South Dakota; Tennessee; Texas; Utah; Vermont;
Virginia; Washington; West Virginia; Wisconsin; Wyoming., has ballpark
transformation

PROJ string:
+proj=pipeline
  +step +proj=axisswap +order=2,1
  +step +proj=unitconvert +xy_in=deg +xy_out=rad
  +step +proj=tmerc +lat_0=0 +lon_0=-53 +k=0.9999 +x_0=304800 +y_0=0
+ellps=GRS80

I could imagine that the reason is that the bbox does not intersect with
the area of use of CGVD28 heigh (despite of the geoid model file covers
that point). Is there a way to workaround it?

Thanks

On Thu, 21 Dec 2023 at 10:27, Javier Jimenez Shaw <j1 at jimenezshaw.com>
wrote:

> Hi
>
> I'm trying to convert from WGS84 (or NAD83, I do not mind)  to "MTM zone 1
> + CGVD28 height" in Canada.
> I know that the area of use of "MTM zone 1" and "CGVD28 height" do not
> intersect. But they told me "Canadian agencies still require this datum [CGVD28]
> throughout all of Canada." Actually the geoid model grid file [ca_nrc_HT2_1997.tif:
> Canada, Natural Resources Canada, NAD83(CSRS)v2 (EPSG:8235) to CGVD28
> height (EPSG:5713)] covers much more than the CRS area of use.
>
> projinfo says something that makes sense to me:
>
> PROJ_NETWORK=ON projinfo -s EPSG:4979 -t EPSG:32181+5713 -o proj
> --spatial-test intersects
>
> Candidate operations found: 303
> -------------------------------------
> Operation No. 1:
>
> unknown id, Inverse of NAD83 to WGS 84 (6) + NAD83 to NAD83(CSRS)v2 (1) +
> NAD83(CSRS)v2 to CGVD28 height (1) + Inverse of NAD83 to NAD83(CSRS)v2 (1)
> + MTM zone 1, 4.55 m, unknown domain of validity
>
> PROJ string:
> +proj=pipeline
>   +step +proj=axisswap +order=2,1
>   +step +proj=unitconvert +xy_in=deg +xy_out=rad
>   +step +inv +proj=vgridshift +grids=ca_nrc_HT2_1997.tif +multiplier=1
>   +step +inv +proj=hgridshift +grids=ca_nrc_NA83SCRS.tif
>   +step +proj=tmerc +lat_0=0 +lon_0=-53 +k=0.9999 +x_0=304800 +y_0=0
> +ellps=GRS80
>
> But cs2cs is not doing any vertical transformation:
>
> echo 47.741422 -53.613219 0 | PROJ_NETWORK=ON cs2cs EPSG:4979
> EPSG:32181+5713 --3d
> 258814.79 5289330.04 0.00
>
> Running with PROJ_DEBUG=3 it ends with
> Using coordinate operation Inverse of Transformation from CGVD28 height to
> WGS 84 (ballpark vertical transformation, without ellipsoid height to
> vertical height correction) + Inverse of Ballpark geographic offset from
> NAD83 to WGS 84 + MTM zone 1
> that makes sense with the no-change in elevation.
>
>
> Why is that happening?
>
> I do not care so much about the version of NAD83. I tried with some and I
> get the same result.
> The same with the source CRS. I use WGS84, but can be any flavour of NAD83.
>
> Thanks.
> .___ ._ ..._ .. . ._.  .___ .. __ . _. . __..  ... .... ._ .__
>
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