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I have some low-quality vector data and I'm trying to align some higher-quality raster images with it so that I can improve the vectors. Unfortunately the rasters have no control points or coordinates, so I have to align the images by matching up two identifiable points that exist in both the raster and the vector:

Raster at left, vector at right, with corresponding points identified

In my 3D software, I can align to these two points without distortion simply by locking the aspect ratio, to get this result:

Desired alignment

But using the Qgis Georeferencer with the same points, I get a distorted result (note that the raster is now considerably less wide left-to-right than the vector):

Distorted output from Georeferencer

I tried different transformation types in the georeferencer and none of them gave the desired result. I kept the CRS set to the same one as the project (WGS84).

How can I align the raster in QGIS with a simple rotate and scale, without altering the aspect ratio?

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Problem resolved. As it turns out there were a couple different things going on:

  • The rasters seem to be projected with a different CRS than my project. I set the project CRS to the appropriate UTM zone and found that the vector distortion was less pronounced, allowing for a better alignment.
  • Additionally, the raster I was working from was just not very good, as I found when I started trying to align it with satellite imagery.

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