[PROJ] Helmert and coordinate convertion question

[Søren Holm]

Thanks for the answer Kristian

It does make sense, however I do not think it is what I want.

I imagined something like this: WGS84 -> ETMERC -> HELMERT

Funny thing is that a 2D helmert works just fine:

+proj=etmerc .... +step +proj=helmert +convention=coordinate_frame ... 
+theta=12345

A 3D helmert does not work - it just complains that the units are 
mismatching.

Why do 2D helmert work when 3D helmert fails? - they both require 
cartesian coordinates. Etmerc outputs projected coordinates which makes 
it even more strange. The documentation does not say much about 
projected coordinates and why they are different from cartesian.

Den 16.11.2022 kl. 12.56 skrev 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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