[PROJ] Vertical CRS transformations and vertical grid shift

[Daniele Romagnoli]

Hi list,
I have started playing a bit with some Vertical CRS transformations and I'm
trying to figure out how they works so sorry in advance if I ask something
obvious.

I have installed PROJ 7.0.0 on an Ubuntu 19.10 using conda and I have
configured the PROJ_LIB to point to the 2 directories:

proj-data-1.0-1/share/proj
as well as
proj-7.0.0/share/proj

so that it should find all the needed definitions.
Moreover I have also copied there a couple of .csv and grid***.tif to
extend the EPSG list.

Then, I started some sample calls to figure out how it works:
projinfo -s "EPSG:5778" -t "EPSG:9274"

It reports "Candidate operations found: 2"
But for the second one I see this message:

Operation No. 2:
EPSG:9275, GHA height to EVRF2000 Austria height (1), 0.05 m, Austria
PROJ string:
Error when exporting to PROJ string: Unimplemented

WKT2:2019 string:
COORDINATEOPERATION[".... long WKT here...."]

What that PROJ Error means? Will it not be able to do the transform?

Then, I tried combining 2DCRS + VerticalCRS:

projinfo -s "EPSG:31255+5778" -t "EPSG:3035+9274"

And I get again 2 candidate operations and long PROJ and WKT Strings.
Now, when I try cs2cs to transform a coordinate I'm getting this:

cs2cs "EPSG:31255+5778" "EPSG:3035+9274"
88000 273000 600
Results into:
*            *        inf

Not really sure what it means. :-(
If I try a different vertical CRS

cs2cs "EPSG:31255+5778" "EPSG:3035+5621"
88000 273000 600
Results into:
2553923.81 4852657.42 600.00

So, at least the 2D coordinates get properly transformed but the elevation
seems unchanged, therefore I'm not sure how it works.

Setting PROJ_DEBUG=2 I see this message running the above cs2cs command:





*pj_open_lib(at_bev_AT_GIS_GRID.tif): call
fopen(/opt/conda/pkgs/proj-data-1.0-1/share/proj/at_bev_AT_GIS_GRID.tif) -
succeededpj_open_lib(GV_HoehenGrid_V1.csv): call
fopen(/opt/conda/pkgs/proj-data-1.0-1/share/proj/GV_HoehenGrid_V1.csv) -
succeededpj_open_lib(GV_Hoehengrid_plus_Geoid_V3.csv): call
fopen(/opt/conda/pkgs/proj-data-1.0-1/share/proj/GV_Hoehengrid_plus_Geoid_V3.csv)
- succeededUnrecognized vertical grid formatvgridshift: could not find
required grid(s).Pipeline: Bad step definition: inv (failed to load datum
shift file)*

That being said, I git cloned the PROJ repo and I have a couple of
questions:
- how the vertical shift get actually computed when grid shift files are
available?
I mean, I suppose that when finding a grid_transformation entry in the EPSG
DB and looking for the vertical_offset_file associated to a file, it will
somehow load that file and use it for a kind of interpolation/pixel query
but I didn't figure out where this happens in the code (checking a bit the
vgridshift.cpp as well as coordinateoperation.cpp).

- Does it support at the same time csv / tiffs / gtx? (If so, I didn't
manage to find where it eventually loads them)

Thanks for your feedbacks in advance and sorry if any of my question
doesn't make sense.

Best Regards,
Daniele


-- 
Regards,
Daniele Romagnoli
==
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Ing. Daniele Romagnoli
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Italy
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