QGIS GeorefExtension Plugin

This is a QGIS Python plugin to extend the features of the QGIS Raster Georeferencer. It's tested in QGIS >=3.24 Windows.

Why do we need a Georeferencer Extension?

The QGIS Georeferencer is a helpful tool to georeference all kinds of raster images, but there are a few features missing that could improve the process of georeferencing.

The following list shows the features I missed when I started using the QGIS Georeferencer more often (that's why I decided to extend it).

1.) Exporting to Virtual raster file (VRT) instead of GeoTIFF.

2.) Iterative improvement of the georeferencing result by repeatedly changing/adjusting GCPs and refreshing the result image

3.) Clipping the georeferenced image with a QGIS feature geometry

4.) Retaining the clipping boundaries for easier creation of image footprints

5.) Changing the background color of the Georeferencer Map Canvas

6.) Quick removal of all GCPs

I will explain each list item later.

What does the extension look like?

The extension consists of an additional Georeferencer toolbar with 3 buttons and a dialog for the VRT options:

	Set Background Color
	Delete all GCPs
	Create Virtual Raster

The "Create Virtual Raster" dialog is only shown if a raster is loaded into the Georeferencer Map Canvas and the button "Create Virtual Raster" is pressed.

How does the Georeferencer Extension work?

The Georeferencer Extension uses GDAL's Python bindings (gdal.translate, gdal.warp) to re-project a raster image using the ground control points (GCPs) from the GCP table, crop the result if necessary and save to VRT.

Since GDAL has no option to define a raster transformation typ, the QGIS Georeferencer transformation type setting (see Georeferencer > Settings > Transformation Settings... > Transformation Parameters) is ignored. GDAL selects the transformation type itself, depending on the number of GCPs. Most, if not all, Transformation Settings parameters are ignored by the Georeferencer Extension, but you have to choose the right Source-CRS and Target-CRS before you start to pick GCPs.

What use cases are there?

Now back to the list of missing features I mentioned earlier.

1.) Exporting to Virtual raster file (VRT) instead of GeoTIFF:

I always found it very impractical that the original images were duplicated and I had to accept a certain loss of quality in the result. In addition, QGIS locks the result file, so I cannot simply overwrite it. This brings me to item 2 of my list.

2.) Iterative improvement of the georeferencing result by repeatedly changing/adjusting GCPs and refreshing the result image

Virtual raster files (VRT) can be overwritten at any time, making iterative georeferencing easy. By iterative georeferencing, I mean repeatedly adding and removing GCPs and refreshing the resulting image in QGIS. This means that the image does not lose any blending mode settings that may have been set and retains the correct layer order.

3.) Clipping the georeferenced image with a QGIS feature geometry

The georeferencing of images very often also requires cropping of the image content. To make this process as efficient as possible, I have created the option of specifying WKT strings as clipping boundaries. This allows us to select individual features using the QGIS Copy Features command and paste the WKT string into the "Enter Cutline WKT" edit field.

4.) Retaining the clipping boundary for easier creation of image footprints

To retain the clipping boundary (cutline), the Georeferencer Extension stores the WKT string inside the Metadata section of the VRT.

  <Metadata>
    <MDI key="CUTLINE">POLYGON ((3335.11 341739.01,4186.13 340840.15,3518.00 340207.58,2666.98 341106.44,3335.11 341739.01))</MDI>
    <MDI key="EXIF_ColorSpace">65535</MDI>
    <MDI key="EXIF_DateTime">2007:09:03 14:43:32</MDI>
    <MDI key="EXIF_Orientation">1</MDI>
    <MDI key="EXIF_PixelXDimension">1280</MDI>
    <MDI key="EXIF_PixelYDimension">1024</MDI>
    <MDI key="EXIF_ResolutionUnit">2</MDI>
    <MDI key="EXIF_Software">Adobe Photoshop 7.0</MDI>
    <MDI key="EXIF_XResolution">(100)</MDI>
    <MDI key="EXIF_YResolution">(100)</MDI>
  </Metadata>

5.) Changing the background color of the Georeferencer Map Canvas

It's sometimes helpful to change the background color of the Map Canvas if we want to select the corners of an image with white background.

6.) Easy removal of all GCPs

Delete all GCPs with the click of a button.

The Create Virtual Raster dialog

How to improve PDF image quality?

If we need to improve PDF image quality, we can change the resolution using the GDAL System Environment variable GDAL_PDF_DPI:

set GDAL_PDF_DPI=300 (Windows). The default value is 250.

Want to crop an already georeferenced Image?

We can use the Georeference Extension to crop already georeferenced images as well. We can drag & drop a GeoTIFF or GeoPDF into the Georeferencer Map Canvas and call Create Virtual Raster without specifying any GCP.

Credit

Credit goes to Enrico Ferreguti (I used his method from changeDataSource plugin to update a VRT raster layer) and to Even Rouault (for his tireless willingness to help with GDAL, i.e. https://trac.osgeo.org/gdal/ticket/6648)