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?– nhoptonCommented Jul 21, 2014 at 9:57
-
Can you add a screenshot of the georeferencer dialogue?– AndreJCommented Jul 21, 2014 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.– user2898914Commented Jul 21, 2014 at 10:14
Add a comment
|
1 Answer
I assume two things causing your described error.
- 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.
- 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.
- 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? Commented Jul 21, 2014 at 9:39
-
1All control points MUST be in the same CRS. Otherwise georeferencing will not work.– AndreJCommented Jul 21, 2014 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.– AndyBCommented Jul 21, 2014 at 12:18