[PROJ] Newbie cs2cs question

[Javier Jimenez Shaw]

Hi

Something I learned this year is that those transformations are not like a
"conservative field". Many times there are "frictions", approximations,
that produce the effect you noticed. In the EPSG you can find many of those
transformations. But there is not always a direct transformation from A to
C (or different options). Definitely there are different precisions. PROJ
does its best to use the proper transformation(s), but it is not easy.
Including concatenating some of them.

AFAIK, PROJ4 always used WGS84 as the pivot CRS. Now it is more complex.

Maybe you can find more info in
https://proj.org/operations/operations_computation.html

Cheers.
.___ ._ ..._ .. . ._.  .___ .. __ . _. . __..  ... .... ._ .__
Entre dos pensamientos racionales
hay infinitos pensamientos irracionales.



On Sat, 5 Sep 2020 at 01:24, <henning at kaarta.com> wrote:

> Hi all,
>
> New to PROJ, using 7.1.1 on linux, still trying to learn the basics.
>
>
>
> So it seems to me that one should get the same results from
>
> cs2cs +init=<crs1> +to +init=<crs2> | cs2cs +init=<crs2> +to +init=<crs3>
>
> as
>
> cs2cs +init=<crs1> +to +init=<crs3>
>
>
>
> i.e. a transform to an intermediate CRS shouldn’t affect the final result,
> but it seems in my case at least it does.
>
> E.g.
>
>
>
> cs2cs -d 6 +init=epsg:4326 +to +init=epsg:7856+5711
>
> 151 -33 5
>
> 313152.777214   6346936.495810 -20.742001
>
>
>
> vs.
>
>
>
> cs2cs -d 6 +init=epsg:4326 +to +init=epsg:32756 | cs2cs -d 6
> +init=epsg:32756 +to +init=epsg:7856+5711
>
> 151 -33 5
>
> 313153.271255   6346937.914960 -20.853001
>
>
>
> Close, but off by about 1.7m. How would I know which one is correct, and
> why are they different?
>
> I must be missing something…thanks!
>
>
>
>
>
>
>
>
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