I am trying to create with PROJ a custom CRS in QGIS 3.2.0 - but I fail.

I have a local, cartesian coordinate system and a transformation report (2D-Helmert) with the following parameters: translation of the x-axis, translation of the y-axis, scale factor in ppm and rotation angles. With this Helmert-transformation the local coordinates should be transformed into cartesian WGS84 coordinates.

So I am trying something like this: proj=helmert x= 2956.5331 y= 5917.9548 z= -28.7120 rx= -0° 00' 09.19941" ry= -0° 00' 02.81345" rz= -1° 39' 10.90398" s= 1 + 0.1347 ppm

I get an error message - am I doing something completely wrong?

Glad about any answers...

Best Susanne

  • That's not a 2D Helmert, but a 3D Helmert. It's normally used in XYZ space. Try using +TOWGS84=2956.5331,5917.9548,-28.7120,-9.19941,-2.81345,-5950.90398,0.1347 but the input data must be in lat/lon or maybe XYZ (3D Cartesian) not local projected coordinates. – mkennedy Aug 8 '18 at 20:11

Yes, some things are wrong, see https://proj4.org/operations/transformations/helmert.html

  • parameters should start with a +
  • +convention is required (PROJ 5.2 onwards)
  • rotations must be in arc seconds
  • proj.exe expects latlon input, so you have to use cct
  • you have to use 3D coordinates x y z for input

So you can try:

 cct -o cctout.txt +proj=helmert +convention=coordinate_frame +x=2956.5331 +y=5917.9548 +z=-28.7120 +rx= 9.19941 +ry=-2.81345 +rz=-5950.90398 +s=1000000.1347 HelmertIn.txt

Sad news at last: This works only in PROJ 5.2, not (yet) within GDAL and QGIS.

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