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. _______________________________________________ PROJ mailing list [email protected] https://lists.osgeo.org/mailman/listinfo/proj
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