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

[Andrew C Aitchison]

On Mon, 17 Oct 2022, Diogo wrote:

> Hi Andrew. Thanks for taking the time to read my email and reply it.
> I'm aware of Copernicus land and other datasets like SMRT3 and ASTER
> however I need the best possible elevation resolution available. To do so,
> I'm getting the elevation dataset directly from countries geographic
> institutes and converting them into the coordinate system 4326+3855. But as
> described in my previous email, I'm having some trouble with some geoids.
> Do you have an idea why I'm having these transformation issues? To be
> honest, I'm a bit lost because I can't understand it.

Sorry, I know no more (and probably less) than you about the transforms.

>From what others have said, I suspect that there is no *meaningful*
way to have better elevation resolution over such a large area than the 
Copernicus data.

I do know that 4326 uses a geoid which is less accurate *over Great 
Britain* than the one the Great Britain Ordnance Survey have been
using for a century or two (OSGB36 Datum 1936, Airy Spheriod 1830!).
I suspect that other national geo-organisations do the same thing and you 
will lose the extra accuracy in standardising on one coordinate system over 
Europe.


> Thanks again for your help,
> Diogo
>
> On Fri, 14 Oct 2022 at 15:57, Andrew C Aitchison <andrew at aitchison.me.uk>
> wrote:
>
>> 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
>>
>

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