4

I am trying to reference an image to a map using the georef plugin. We have images from electrical resistivity tomography which are normal pngs with image coordinates. and we have measured points in the field by an RTK GPS measurement. What I need is a linear referencing of the image (only worldfile) with shift, scale and rotate. The image below shows the situation:

enter image description here

The result is anywhere but not on the section line. the worldfile result is like this:
0.02247045541586595
0
0
-81.09547939153824814
412331.87089667085092515
4385191.09988579899072647

as you see, there is no rotation, it should be like this - done with ArcGIS reference tool with 2 points:
-0,022802199437638223
-0,036435520886296563
-0,036435520886296563
0,022802199437638223
412333,69858059636

what I am doing wrong? any hints or help would be great, because we want to move completly from ArcGis to QGIS...

3

As far as I know you can't rotate and use the Linear transformation (which lets you keep the original raster with a world file). You need Helmert or a higher order algorithm with 3 or more control points; that ends up modifying the raster and saving as geotiff. What I usually do if I need a wld file is save the rotated geotiff to the appropriate format - edit out the "black" sections from rotating if required - and re-reference it with a linear algorithm, resulting in a wld file.

2
  • 1
    IMO rotation should be possible using linear transformation. The wolrd file has the possibility to set rotation parameters (Line 2 and 3 in worldfile: de.wikipedia.org/wiki/World_file). Since I did this all the time with arcgis it should be also possible in QGIS. – eurojam Aug 3 '17 at 5:18
  • It should be possible but alas I haven't had any luck doing it in QGIS. Some browsing around suggests ArcGIS only started supporting rotation parameters from 2011 onwards and QGIS has yet to support it because support on other platforms is inconsistent. From 2011: gis.stackexchange.com/questions/12181/… lists.osgeo.org/pipermail/qgis-user/2011-November/014188.html "World-file export is currently not enabled for helmert+1st order because rotated world-files are not always well-supported, even by qgis." – she_weeds Aug 7 '17 at 1:53

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