[PROJ] System34 height difference

[Søren Holm]

Thank you for the answers Kristian.


søndag den 26. april 2020 15.11.30 CEST skrev Kristian Evers:
> 
> Hi
> 
> We are using proj for transformation of some Danish coordinatesystems.
> 
> Among them are S34 and DKTM.
> 
> Usually the height is not affected much during transformations. This is
> however different for S34.
> 
> 
> Yes, System34 is only a 2D coordinate reference system, so the height is
> generally not meant to be used.
 
> 
> $ cct -t0  +proj=pipeline +step +init=DK:DKTM1
> 10 55 0 0
>  263991.7405   1097565.8295        0.0000        0.0000 0
> 
> 
> $ cct -t0  +proj=pipeline +step +init=DK:S34J
> 10 55 0 0
>  234033.0592     63129.6022      -36.8103        0.0000 0
> 
> 
> Why is there ~36m subtracted for S34?
> 
> The reason is that there is an implicit datum change happening in that
> transformation, which internally works in three dimensions and then
> consequently transforms the zero height to another datum. This, however,
> has no physical meaning and can be regarded as a mathematical curiosity.
> With PROJ > 6.0.0 this can actually be avoided by using the
> push<https://proj.org/operations/conversions/push.html> and
> pop<https://proj.org/operations/conversions/pop.html> operators:
 
> echo 10 55 | ./bin/cct -z0 -t0 +proj=pipeline +step +proj=push +v_3 +step
> +init=DK:S34J +step +proj=pop +v_3
 234033.0592     63129.6022       
> 0.0000        0.0000
> 
> The DK init file was created before that was possible. This probably should
> be updated, although it comes with the risk that someone now relies on the
> current behaviour. System34 has been submitted to the EPSG but the
> application is still pending. Once that find it’s way to the EPSG registry
> this problem should disappear entirely in PROJ. The time frame is
> unfortunately unknown at this time.
 
> Why is there around 0.1mm height difference for most UTM and NTM zones?
> 
> I haven’t been able to replicate that, so hard to comment on. A guess is
> that there is some numerical inaccuracy involved. I don’t think there’s
> anything to worry about, 0.1 mm is very likely less than both the accuracy
> of the data and the transformation.
 
> Is it the intention that the heights should be affected during these
> transformations?
> 
> For System34 it’s yes and no. Yes, because that is the way the
> transformation is currently made but no since it the height isn’t really
> part of the system. Can’t comment on the other case.
 
> /Kristian
> 


-- 
Søren Holm