[PROJ] Vertical CRS transformations and vertical grid shift

[Daniele Romagnoli]

On Thu, Apr 16, 2020 at 5:15 PM Even Rouault <even.rouault at spatialys.com>
wrote:

> > Well, when providing that coordinate, I got them from a sample Dataset in
>
> > EPSG:31255 so they should be valid.
>
> >
>
> > Weird thing:
>
> > cs2cs "EPSG:31255" "EPSG:4326"
>
> > 88000 273000
>
> >
>
> > 45d52'39.123"N 16d50'57.28"E 0.000 (as you said)
>
> >
>
> > However:
>
> > gdaltransform -s_srs EPSG:31255 -t_srs EPSG:4326
>
> > 88000 273000
>
> >
>
> > 14.5025684938377 47.5898013589131 46.4711159374565
>
> >
>
> > And same thing if I draw a point in QGIS: SRID=31255;POINT(88000 273000)
>
> > and I switch the map projection to 4326, that rendered point on the map
> has
>
> > the same coordinates reported by gdaltransform.
>
> >
>
> > What do you think about it?
>
>
>
> Ah, the reason is axis order. EPSG:31255 uses Northing,Easting, and cs2cs
> honours it. Whereas gdaltransform uses the "traditional GIS axis order", ie
> easting, northing.
>
>
>
> So with PROJ master:
>
>
>
> $ echo 273000 88000 600 | PROJ_NETWORK=ON src/cs2cs "EPSG:31255+5778"
> "EPSG:3035+9274"
>
> 2729881.62 4659643.84 599.75
>

Ah. That makes sense. Thanks.
Final question (sorry about that):
Are you aware of any involvement of this kind of reprojection in gdal_warp
processing?
I mean, I just did a test by building PROJ master and GDAL 3.1 against that
PROJ version and tried a

gdalwarp -s_srs "EPSG:31255+5778" -t_srs "EPSG:3035+9274" dem.tif
warpeddem.tif

Is it expected to reproject the DEM values? :-)
Asking that since I'm trying to figure out a way to see that vertical
transformation used in some practical use-case with DEM raster but I didn't
get the pixel values modified so I probably went too far and that's not how
it should be used.

Thanks again for your feedbacks so far.
Regards,
Daniele


>
>
> With EPSG:5621 "EVRF2007 height" instead of EPSG:5778 "EVRF2000 height", I
> don't get any change in Z, because there's no registered transformation
> between EPSG:5778 "GHA height" and it.
>
>
>
> Even
>
>
>
> --
>
> Spatialys - Geospatial professional services
>
> http://www.spatialys.com
>


-- 
Regards,
Daniele Romagnoli
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Ing. Daniele Romagnoli
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