[PROJ] Problems with CGVD28 height

[Even Rouault]

I've already contacted NRCan about that. They're investigating.

Even

Le 27/12/2023 à 19:36, Greg Troxel via PROJ a écrit :
> You said that you get the same results starting from NAD83.  I would
> suggest debugging the transform NAD83 and not trying to use WGS84 until
> the first step works right.  proj is known to mistransform WGS84 to
> NAD83 due to ensemble fuzz.  My understanding is that NAD83(CSRS)v2,
> being from 1998, will be pretty closely aligned with newer US NAD83
> realizations and later Canadian ones, and thus not have the accuracy
> issues of the 1986 version of NAD83, which is what causes 90%+ of the
> issues.  So this is all likely a red herring, but still avoiding asking
> proj to do it will avoid trouble; you don't need that trouble and height
> trouble both at the same time.
>
> NRCAN seems to publish transforms from ITRF to modern NAD83(CSRS).
> So I'd assume your "WGS84" coordinates are ITRF, use the NRCAN
> transforms, and then assume that those coordinates in some highish
> version are equivalent to v2, as a first guess.
>
> Looking at EPSG:32181, MTM zone 1, the bbox shown is enormous, but the
> description says: Area of use: Canada - Newfoundland - onshore east of
> 54°30'W.
>
> Looking at EPSG:5713, CGVD28, it seems that Newfoundland is not part
> of it, and it starts at -59.73.   I have to wonder if that is intended
> or a bug.  It is derived from leveling, and if they didn't have leveling
> to Newfoundland, it just wouldn't be part of the datum.   It seems the
> grid files are probably some modern estimation of what the heights would
> have been had they been defined.
>
> If you get datasheets for passive control in eastern Newfoundland, do
> they publish CGVD28 heights?
>
> I went to
>
>    https://webapp.csrs-scrs.nrcan-rncan.gc.ca/geod/tools-outils/gpsh.php?locale=en
>
> and entered rough coords for St. John's
>
>    latitude 47.5825
>    longitude 52.8580 (west)
>    h 0
>
> for CGVD28 HT2_2010v70 and NAD83(CSRS) [they don't specify versions
> which is an accuracy clue], then I get -10.121 for H.  On the other hand
> if I put in Montreal coordinates
>
>    latitude 45.5095
>    longitude 73.5768
>    h 0
>
> I get H 30.78.
>
> So they are returning results in their web tool.
>
>
> Given all of that, the comments from Erixen Cruz make a lot of sense to
> me.  If they've published a grid file covering newfoundland, and it has
> sensible values, then it may make sense to use it.  The flip side is
> that the area of validity of a datum is not a bbox in lat/lon.  The
> shape can be much more complicated.  So it's not necessarily possible to
> publish a grid file that only covers the valid area, unless it has
> codepoints for invalid.  I bet the senior height expert at NRCAN knows
> how they are trying to thread this needle.
>
> I would write to NRCAN and ask them.  I have very little experience with
> them, but whenever I have asked what I thought was a a hard question to
> NGS, only a small number of times, I have gotten a really good answer
> from someone whose name I recognize from reports.
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