[PROJ] Problem understanding units - UTM->helmert

[Kristian Evers]

Søren,

In the context of PROJ (and geodesy in general) cartesian coordinates refer to 3D coordinates with the origin in the center of the earth. Earth-centered coordinates is another term that is sometimes used. I realize this can be confusing since the coordinates of a UTM projection are also cartesian although only a 2D coordinate system.

See https://proj.org/operations/conversions/cart.html for more on the cartesian operation. 

You probably want to do the helmert operation before converting to UTM. Something like this would be more in the lines of what you are describing:

+proj=pipeline +step +proj=cart +ellps=WGS84 +step +proj=helmert ... +step +proj=cart +ellps=WGS84 +inv +step +proj=utm +zone=32

/Kristian

> -----Original Message-----
> From: PROJ <proj-bounces at lists.osgeo.org> On Behalf Of Søren Holm
> Sent: 14. juni 2021 09:01
> To: proj at lists.osgeo.org
> Subject: [PROJ] Problem understanding units - UTM->helmert
> 
> Hi
> 
> I'm doing a pipeline of going from WGS84->UTM32->helmert but I have
> difficulties understanding the errors that proj gives me.
> 
> $ cct -t0 +proj=pipeline +step +proj=utm +zone=32 +ellps=GRS80
> +datum=WGS84  +no_defs   +step +proj=helmert
> +convention=coordinate_frame
> 
> proj_create: Error 1025 (Invalid PROJ string syntax): pipeline: Pipeline:
> Mismatched units between step 1 and 2
> 
> 
> However if I add fx. "+theta=5" to the proj string there is not mismatching
> units.
> 
> UTM documentation states its output as "Projected coordinates"
> Helmert states its input as "Cartesian coordinates"
> 
> What are the differences? and why do helmert work in "2D mode" using
> theta and not in 3D ?
> 
> Thanks in advance :)
> 
> --
> Søren Holm
> 
> 
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