[Proj] converting WGS84/ETRS89 to ITM/OSGM15

[Elmir Jagudin]

On Wed, Aug 22, 2018 at 12:12 PM Alain Orsoni <Alain.Orsoni at ign.fr> wrote:

> Hi,
>
> Trying to answer with my knowledge :
> On my computer I can do this transform in this way
>
> echo -6.6 53 21 | cs2cs +init=epsg:4326 +to +proj=tmerc +lat_0=53.5
> +lon_0=-8 +k=0.99982 +x_0=600000 +y_0=750000 +ellps=GRS80
> +towgs84=0,0,0,0,0,0,0 +geoidgrids=egm08_25_rg.gtx +units=m +no_defs
> 693972.63 695281.52 -35.37
>
> The result is a little bit different from what you expect. Maybe because I
> use egm08 and you expect another Geoid model.
>

Aha, cool. This looks like the right path for me.

Yes, the OSGM15 is probably using slightly different geoid model. I'll try
to create gtx file for OSGM15.

If anyone is curious, there is a document the describes OSGM15 here:
https://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/formats-for-developers.html

/Elmir
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