[gdal-dev] Transformations with same horizontal datum but different geoid outputs the same Z value

[Andrew C Aitchison]

On Fri, 14 Oct 2022, Diogo wrote:

> Hello everyone!
>
> I'm merging multiple elevation datasets from different countries and I
> would like to re-project all into the same coordinates system, 4326+3855
> using gdalwarp. I'm using `gdaltransform` to confirm that there is in fact
> a conversion from the original projection to the aspected one. However, for
> some datasets, there is only conversion in the XY axis, while the Z output
> is the same as the input.

Is there a good reason to start with these particular datasets ?

There are international bodies who have produced elevation datasets covering the whole of 
Europe, eg
https://land.copernicus.eu/user-corner/how-to-access-our-data
https://www.eea.europa.eu/data-and-maps/data#c0=5&c11=&c5=all&b_start=0&c12=elevation
https://www.eea.europa.eu/data-and-maps/data/copernicus-land-monitoring-service-eu-dem

which would save you trying to stitch together data in different CRSes.

> Below there are some examples (the geoid code is extracted from the dataset
> documentation)
>
> Genoa 1942 height (Italy)
>
> $ echo "14.995 37.755 3357" | PROJ_NETWORK=ON gdaltransform --debug on
> -s_srs "EPSG:4326+3855" -t_srs "EPSG:32632+5214">> 1028333.73796622
> 4195591.29690422 3357
>
> N2000 (Finland)
>
> $ echo "25.15296 62.50195 269" | PROJ_NETWORK=ON gdaltransform --debug
> on -s_srs "EPSG:4326+3855" -t_srs "EPSG:3067+3900">> 404861.868388335
> 6931460.32557422 269
>
> EVRF2007 (Estonia)
>
> $ echo "27.059167 57.714444 318" | PROJ_NETWORK=ON gdaltransform
> --debug on -s_srs "EPSG:4326+3855" -t_srs "EPSG:3301+5621">>
> 682284.606829111 6401087.4159131 318
>
>
> I don't have much experience with the projections and I'm a bit lost
> to understand the possible issues, or interpreted the Z output. What
> can be the possible issues?
>
> Any help would be appreciated.
>
> Best regards,
>
> Diogo
>

-- 
Andrew C. Aitchison                      Kendal, UK
                    andrew at aitchison.me.uk