[PROJ] Helmert and coordinate convertion question

[Kristian Evers]

Søren,

You simply add a step with the cart operation:

+proj=pipeline 
+step +proj=etmerc +ellps=GRS80 +lat_0=0 +lon_0=20.25 +k=1 +x_0=150000 +y_0=0 +inv
+step +proj=cart +ellps=GRS80
+step +proj=helmert +convention=coordinate_frame +...

I've removed superfluous parameters in the etmerc step and added a "+inv". The last bit
ensures that correct direction of the etmerc step. You wan't to go from projected
coordinates to geodetic coordinates. The cart step then converts the geodetic coordinates
to cartesian coordinates which can be consumes by the helmert operation.

You probably want to add more steps to bring your coordinates to a more usuable format
than cartesian, e.g.

+step +proj=cart +ellps=GRS80 +inv
+step +proj=....

Hope that clears things up.

/Kristian

> -----Original Message-----
> From: PROJ <proj-bounces at lists.osgeo.org> On Behalf Of Søren Holm
> Sent: 15. november 2022 16:00
> To: proj <PROJ at lists.osgeo.org>
> Subject: [PROJ] Helmert and coordinate convertion question
> 
> Hi
> 
> I'm scratching my head here.
> 
> How do I convert the projected coordinates from the etmerc step into
> cartessian coordinates for the helmert step ?
> 
> cct +proj=pipeline +step +proj=etmerc +ellps=GRS80 +lat_0=0 +lon_0=20.25
> +k=1 +x_0=150000 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m
> +no_defs +step +proj=cart +ellps=GRS80 +step +proj=helmert
> +convention=coordinate_frame
> 
> 
> Thanks in advance.
> 
> /Søren Holm
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