[PROJ] WGS84/NAD83 Original to {current} transforms

[Even Rouault]

Matt,

> 
> Within the Yukon and NWT (Canada), how do you convert spatial data
> horizontal datum from NAD83-Original (circa 1994) to NAD83-CSRS (2002+)?

I suppose this is the same topic as what you've revived recently in
https://github.com/OSGeo/PROJ/issues/202#issuecomment-556545661 ?

> 
> I’ve been up and down NRCAN’s geodetic reference site[0] and a bunch of
> other ones and am more confused than when I started. The consensus on the
> net seems to be to use NTv2 to migrate from Original to CSRS, but there are
> no grid shift files for the territories [1],[2].
> 
> Esri (ArcGIS Desktop) and Safe (FME) use transforms which are a null
> operation, all that happens is a change in the name, the coordinates are
> left unaltered. However looking at the v7 velocity grid[3] it looks like
> there have been significant shifts since the 1990s.
> 
> Most of our new data acquisitions from survey and LiDAR are coming in as
> CSRS. When combining with our old data and project-on-the-fly I’ve observed
> different coordinates for the same points depending on how the locations are
> reported. So we need to bring our holdings into a common coordinate system
> to be assured our analyses are the most correct they can be, but how?

With PROJ 6.2.1, this should be mostly require to find the grids mentionned in the ESPG transformation records I listed at
https://github.com/OSGeo/PROJ/issues/202#issuecomment-556792769
and putting them in the PROJ_LIB resource directory

For example, if I try

$ projinfo -s NAD83 -t "NAD83(CSRS)v4" --spatial-test intersects -o PROJ

I get:

Candidate operations found: 2
-------------------------------------
Operation n°1:

EPSG:9119, NAD83 to NAD83(CSRS)v4 (9), 0.1 m, Canada - British Columbia - mainland, at least one grid missing

PROJ string:
+proj=pipeline +step +proj=axisswap +order=2,1 +step +proj=unitconvert +xy_in=deg +xy_out=rad +step +proj=hgridshift +grids=BC_93_05.GSB +step +proj=unitconvert +xy_in=rad +xy_out=deg +step +proj=axisswap +order=2,1

Grid BC_93_05.GSB needed but not found on the system.

-------------------------------------
Operation n°2:

unknown id, Ballpark geographic offset from NAD83 to NAD83(CSRS)v4, unknown accuracy, World, has ballpark transformation

PROJ string:
+proj=noop


OK, so after logging, I could download
https://webapp.geod.nrcan.gc.ca/geod/process/download-helper.php?file_id=BC_93_05

Unzip BC_93_05.zip and place the BC_93_05.GSB file in PROJ_LIB directory, and then


echo "50 -120" | cs2cs -d 9 NAD83  "NAD83(CSRS)v4"
50.000000083	-120.000000806 0.000000000

It works !

Ultimately if those grids meet the open source licensing mentioned at

https://github.com/osgeo/proj-datumgrid#about-the-datum-grid-package 

they could be included in the proj-datumgrid-northamerica package.

And apparently, from
https://webapp.geod.nrcan.gc.ca/geod/data-donnees/transformations.php?locale=en
they are subject to Open Government Licence - Canada
https://open.canada.ca/en/open-government-licence-canada

https://opendefinition.org/licenses/ lists it under the "Other conformant licenses" category.
Roughly a Creative-Commons By style.
So could probably be candidate for inclusion in proj-datumgrid.


Even

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