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I have georeferenced some aerial images, loaded in QGIS and saved them (files .tiff, aux.xml and .points). These images overlap, however I found that some details in the borders did not match exactly after georeferencing (perhaps due to the lack of GCPs - I am using TPS transformation). Since I am building a mosaic, I wanted to add a few more GCPs to improve the precision and match the elements in the borders of these images.

In order to do so, I reopened Georeferencer, loaded the unprocessed image (.tiff file) as well as the GCPs previously created (.points file) over that same image. The red GCPs showed on the Georeferencer window, but not on QGIS. When added extra points, they showed on both windows, but I noticed that the calculated residual values disappeared (became zeros) in the GCPs table. From what I understand, no residuals means no georeferencing process. When trying to load the raster into QGIS, there are no error messages, only a grey rectangle on a strange scale, different from the projects CRS.

Is this procedure (adding/editing previously saved GCPs) even possible? What I am doing wrong?

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The process of adding and editing georeferenced points is doable within.

Toggling editing of GCPs should allow you to select a GCP on either the map or on the img to reconfigure placement. Toggling new GCP creation should allow the insertion of a new GCP.

Your problem may be:

  • CRS error between project and img
  • GCP file error
  • within the image (e.g. aerial photo is skewed at the edges, such as from a fish eye lens that a go pro may cause)

There are checks that are possible when you are editing GCPs that you have already marked. If they are not appearing I would advise: settings - configure georeferencer - show ID or coordinates. This will tell you if georeferencer is recognising that the GCPs are present even if they are not directly visible.

If they are not showing I would recommend reloading the GCP file as it may be that this is where the problem is.

Georeferencer can also toggle whether a GCP is visible or not: view - panels - GCP tables (make sure this is ticked). This can then be used in conjunction with known coordinates to determine if any of the GCPs are missing, if there are a large number in use.

May it be that the GCP file that is being loaded was from another georeferenced image that was opened after the first one?

It could also be worth checking the project CRS and img CRS are both the same for GCP clarity.

If the aerial photo is skewed another program may be needed to fix this problem outside of GIS.

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