0

I am very new at this and when I georeference an image in qgis, I get the same image georeference but rotate and elongated.

I use 3 control points to georeference the image but whatever I do, I get the image with the same errors.

  • Is the map image available somewhere for downloading and which coordinate reference system do you wish to use? – nhopton Jul 21 '14 at 9:57
  • Can you add a screenshot of the georeferencer dialogue? – AndreJ Jul 21 '14 at 9:59
  • I don't know if the map is available somewhere, is a historic map. I want to use ETRS89 but the map have ED50/UTM 29N 30N.. coordinate systems. – user2898914 Jul 21 '14 at 10:14
2

I assume two things causing your described error.

  1. Make sure you chose the right in- and output coordinate system. If the input image has a wrong coordinate system the output is adjusted according to these distorted information.
  2. Try to distribute your control points over the whole image. Additionally, if they are too close smaller inaccuracies have a larger effect. If the triangle of your three control points is spread over the whole image, you can minimzie this problem.
  3. Try adding some more control points. Even though the accuracy doesn't benefit linearly from more control points, you can select and deselect some of them and see how the output changes.
  • Ok, I have a little problem. I want to goreference a big image and it have a few coordinate systems. How do I have to do if my control points have differents coordinate systems? – user2898914 Jul 21 '14 at 9:39
  • 1
    All control points MUST be in the same CRS. Otherwise georeferencing will not work. – AndreJ Jul 21 '14 at 11:55
  • I agree to Andre Joost, if the control points do not match the coordinate system of the reference image, the transformation to a target CRS is critical. First, try to find out which coordinate System your map is projected in. gdalinfo is a helpful tool for this purpose. – AndyB Jul 21 '14 at 12:18

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.