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

[Diogo]

Hi there! Thanks, everyone for your help and recommendations provided.



Even, I wasn't aware of that, as many other things I see. I thought that It
could be translated directly

> 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
>
> In that situation, "projinfo -t EPSG:32632+5214 -s EPSG:4326+3855
> --spatial-test intersects" shows a potential transformation, but that
> requires that you need to get (buy) the geo_igm_mar06.grd grid which does
> the "ETRS89 to Genoa 1942 height" transformation from
> https://www.igmi.org/en and convert it into a format understood by PROJ
>

Prefixing your command with "PROJ_NETWORK=ON" outputs a WTK without the
warnings about the missing grid. Should this be enough?

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
>
> There's only one known transformation between EVRF2007 and ETRS89 or
> WGS84, but it is for Slovakia.
>
Could you tell me which one it is? I will need to transform Slovakia as well


Basically, one needs to pick a worldwide height system and then
> transform data from multiple regions into it.  This is hard because
>
[....]
>

Greg, that is exactly what I'm doing but I was not aware of how hard it
could be. I would like to have the final dataset in 4326+3855. Would be
easier to use the EGM96 instead?

Could you please clarify one thing for me?

- It is often hard to find transformations from national height datums
>     to WGS84 Orthometric Height.  In the US, I don't know of any.  But
>     there are published geoid models from NAVD88 to NAD83 HAE, and then
>     one can transform to WGS84(G2159) HAE, and then use EGM2008 to get
>     WGS84 Orthometric Height.
>

Converting for example the USA NED 10M dataset, I can't reproject all
dataset to 4326+3855 without any intermediary transformation step? For
instance, in the below command, isn't the Z output value precise?
$ echo "-112.55260 33.48142 643" | PROJ_NETWORK=ON gdaltransform -s_srs
"EPSG:6318+5703" -t_srs EPSG:4326+3855
>> -112.5526 33.48142 643.369641951764


>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.
>

Andrew, I'm using Copernicus as a fallback when I can't find a dataset with
better resolution. Because I need to get the most accurately possible
elevation data, I'm merging datasets with 10M, 5M, or 2M pixel resolution
with Copernicus data. I'm aware that this is not an easy task, but
currently, my main issue is to understand the particularities of
transforming the original dataset projection into the desired one. Because
my background is not related to GIS makes this task more challenging :'-)

Best regards,
Diogo
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/gdal-dev/attachments/20221018/ef38412d/attachment.htm>