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I want to georeference a map (TIFF) from 1965 using the grid crosses as ground control points. The map is in the local coordinate system which I calculated and wrote a custom coordinate system for.

When I want to start the georeferencing it says transformation is not possible. I am afraid it doesn't work because of my custom coordinate reference system. If I use another system it "works" ( but then the map is wrong obviously).

DERIVEDPROJCRS["Milet lokal 2015 (Bendt)",
    BASEPROJCRS["WGS84 / UTM zone 35N",
        BASEGEOGCRS["WGS 84",
            DATUM["World Geodetic System 1984",
                ELLIPSOID["WGS 84",6378137,298.257223563,
                    LENGTHUNIT["metre",1]]],
            PRIMEM["Greenwich",0,
                ANGLEUNIT["degree",0.0174532925199433]]],
        CONVERSION["UTM zone 35N",
            METHOD["Transverse Mercator",
                ID["EPSG",9807]],
            PARAMETER["Latitude of natural origin",0,
                ANGLEUNIT["degree",0.0174532925199433],
                ID["EPSG",8801]],
            PARAMETER["Longitude of natural origin",27,
                ANGLEUNIT["degree",0.0174532925199433],
                ID["EPSG",8802]],
            PARAMETER["Scale factor at natural origin",0.9996,
                SCALEUNIT["unity",1],
                ID["EPSG",8805]],
            PARAMETER["False easting",500000,
                LENGTHUNIT["metre",1],
                ID["EPSG",8806]],
            PARAMETER["False northing",0,
                LENGTHUNIT["metre",1],
                ID["EPSG",8807]]]],
    DERIVINGCONVERSION["Affine",
        METHOD["Affine parametric transformation",
            ID["EPSG",9624]],
        PARAMETER["A0",-428178.87985533,
            LENGTHUNIT["metre",1],
            ID["EPSG",8623]],
        PARAMETER["A1",1.00014514665939,
            SCALEUNIT["coefficient",1],
            ID["EPSG",8624]],
        PARAMETER["A2",-0.0227055108057824,
            SCALEUNIT["coefficient",1],
            ID["EPSG",8625]],
        PARAMETER["B0",-4164309.21582976,
            LENGTHUNIT["metre",1],
            ID["EPSG",8639]],
        PARAMETER["B1",0.0227055108057824,
            SCALEUNIT["coefficient",1],
            ID["EPSG",8640]],
        PARAMETER["B2",1.00014514665939,
            SCALEUNIT["coefficient",1],
            ID["EPSG",8641]]],
    CS[Cartesian,2],
        AXIS["(E)",east,
            ORDER[1],
            LENGTHUNIT["metre",1]],
        AXIS["(N)",north,
            ORDER[2],
            LENGTHUNIT["metre",1]]]

Also my QGIS version 3.16 is not the problem. I tried with version 3.10.

Is there a way to georeference a TIFF to my custom coordinate system?

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  • What happens if you temporarily overwrite the CRS of the map (of both the layers instances inside the project and the project itself), saying that it is made in UTM zone 35N instead of its 2D affine derived CRS? I think that you will be able to warp the historic map to UTM, and then just inversely overwrite its CRS to say that it is not in UTM but in its 2D affine derived one. Apr 9, 2021 at 6:18
  • I forgot to mention that the project is in (EPSG 32635) UTM35. Only some layers are in the derived affine system. But this doesn't work. If I click Raster->Projection->Transform and try to transform from utm to local, the is an error message; and if I simply change the raster layer ccs from utm to local, the map is shown in the slightly wrong place. Apr 9, 2021 at 8:22
  • 1
    So you have the raster data georeferenced in UTM? You can reproject it via gdalwarp, outside from QGIS. But since the relation between UTM 35N and the local grid system is linear, just don't say the georeferencer that it is a derived CRS, georeference as if it is any standarized CRS and then overwrite it. In any case, better if operations with the derived CRS are done outside QGIS, you can use QGIS to see all data together but not much more yet. Apr 9, 2021 at 20:16

1 Answer 1

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I would try to georeference it to EPSG:4326 and then change that to your local system. A 2-step process. Have you tried it?

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  • Oh this works. But isn't it inaccurate? Apr 9, 2021 at 12:21
  • If it is inaccurate, then you should see that in your control points. If they are correct in both systems, it is probably correct. Your parametric affine transformation is only an approximation anyway.
    – wingnut
    Apr 10, 2021 at 8:29
  • I forgot something obvious. From your raster, make an XYZ dataset. Transform the x and y, but not the z, then regrid. That should be as accurate as your parametric affine transform.
    – wingnut
    Apr 10, 2021 at 8:37
  • And finally, you could just georeference the map to UTM instead. That is, digitise the crosses in local, convert them to UTM, and then use UTM values when georeferencing the map.
    – wingnut
    Apr 10, 2021 at 8:47

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