[PROJ] Problem understanding units - UTM->helmert

[Søren Holm]

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