3

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

| improve this question | |
  • 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
1

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.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.