[PROJ] Fwd: some help with cm-level GRS80 to WGS-84 transformation?

[Greg Troxel]

Joseph Boardman <boardman at aigllc.com> writes:

> I can do the transform I want in the ESRI PE (used by ENVI) and get a
> dx,dy,dz grid much as in the link I just sent (say +/-1m in x,yz
> differences in conus).
>
> But I'd like to do it in Proj if it can.
>
> ENVI> proj1=envi_proj_create(/geographic,datum='WGS-84')
> ENVI> proj2=envi_proj_create(/geographic,datum='North America 1983')

That's making assumptions (perhaps they are reasonable) about which
NAD83 and which WGS84.  But without knowing them, and knowing what
transform it is using, there's no good basis to trust the results.
Perhaps you can find that in the docs.  It's also ignoring epoch, or
perhaps making some assumption.

I do something similar, but with specific realizations, as

  ogr2ogr -f GeoJSON -t_srs "EPSG:7912" -s_srs "EPSG:6319" foo.geojson foo.gpkg foo

which actually invokes proj.  (Actually the -s_srs is not needed as my
geopackage layer has a datum code, but my scripts have been spiffed up
over time.)   And yes, my transform gets me ITRF2014 epoch 2010.0, which
lines up well on webmaps and I have not really figured out further.


Please do report back when you figure out what you really mean and what
you are doing about epoch and why, and how you are aceessing/checking
correctness relative to WGS84.




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