[PROJ] Help with custom CRS proj string

[Javier Jimenez Shaw]

This works for me. See that cct needs 3 coordinates. x,y,z

$ echo 59770.038 101069.654 0 | cct +proj=helmert +x=334491.216
+y=6950332.853 +theta=-49800
368362.0519   7062761.9368        0.0000           inf


On Wed, 21 May 2025 at 00:36, Daniel Farfán via PROJ <proj at lists.osgeo.org>
wrote:

> I've been trying to solve this problem but was unable to, so I kindly
> ask for help.
>
> I have 3 coordinate points in a local grid:
> A': 59770.038, 101069.654
> B': 59214.93435, 98996.58573
> C': 58444.7795, 99231.37298
>
> Corresponding coordinate points in EPSG:32719 CRS:
> A: 368362.117, 7062761.917
> B: 368318.78, 7060616.253
> C: 367514.826, 7060660.088
>
> So, I've determined that to convert  from the local points to the
> EPSG:32719 CRS, I have to displace the x axis by 334491.216, the y
> axis by 6950332.853, and rotate counterclockwise by 13.8333 degrees.
>
> I have managed to get the following to work:
>
> cct +proj=helmert +x=334491.216 +y=6950332.853 +theta=-49800
> When I input the coordinates in the local grid, i get the
> corresponding coordinates in EPSG:32719
>
> What I'm struggling with is to use the conversion with cs2cs: i think
> I need to define the transformation as a custom CRS, to be able to do
> something like:
> cs2cs "+proj=helmert +x=334491.216 +y=6950332.853 +theta=-49800" +to
> EPSG:32719
>
> but that's not working.
>
> My final goal is to use the custom CRS proj string (or WKT equivalent)
> in GDAL and QGIS to be able to transform on the fly the local grid
> data to EPSG:32719.
>
> Any help would be appreciated. Thanks in advance.
>
> --
> Daniel Farfán
>
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