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I am trying to follow this tutorial to convert a JPEG image of a WGS-84 Web Mercator world map to a GeoTIFF file using QGIS, but am failing to do it correctly-- the map comes out with black-hole-like warping:

Original jpg: enter image description here

Geotiff output (transformation type: inverse-spline): enter image description here

I am inputting EPSG:3857 coordinates using this online tool, and in QGIS I set the Target SRS to EPSG:3857.

Here are some debugging steps I tried:

  • Swap lon/lat values to lat/lon: did not fix, lon/lat seems correct
  • Change transformation type to any of the other options (linear, projective, polynomial, etc): they all produce different types of skewing and warping
  • Try adding more points, or spreading them around in different formations: did not fix, just changed the locations of the warp holes

Does anyone have an idea of what I'm doing wrong?

I'm simply trying to generate a raster tileset for Mapbox, which requires a geotiff file, so if anyone would like to suggest any other free methods of doing this for a GIS-newbie, I'd love to hear them!

Edit:

Here's a screenshot of my georeferencer transformation settings: enter image description here

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  • Can you include a picture of the image that you are trying to georeference to? Can you include a screen shot of the transformation setting in the QGIS georeferencer?
    – GBG
    Commented Jun 10, 2022 at 19:46
  • I'm trying to georeference to a mapbox map. So that's just like any interactive map you see online, for example the one embedded on this page: docs.mapbox.com/mapbox-gl-js/example/georeference-imagery
    – A__
    Commented Jun 10, 2022 at 19:51
  • Maybe I should also mention that the coordinates I'm adding to the georeferencer tool aren't exact-- I'm basically eyeing their positions then copypasting the resulting value from epsg.io/map#srs=3857 ...I figured a certain level of exactitude wasn't necessary...
    – A__
    Commented Jun 10, 2022 at 19:58

1 Answer 1

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Alright, so I've finally successfully generated a nice looking GeoTIFF file. I discovered a slightly different tutorial outlining more-or-less the same QGIS process but with a little more detail describing what the dX, dY and Residual error values are, and how to adjust them. I adjusted the points I added to the JPG until all dX and dY error values were as close to 0 as possible. Then I generated the GeoTIFF file with Polynomial 2 transformation type and it came out fine.

So my suspected diagnosis is that I had too-high of error rates previously, although they must have not been very noticeable so it's best to be super precise.

Here is a screenshot showing the successful coordinates and error values:

enter image description here

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    You used originally Thin Plate Spline method that forces the residuals of all ground control points to be zero and image is warped exactly to every GCP . But if the GCPs are way off then the warped image can get really weird shapes as you noticed.
    – user30184
    Commented Jun 10, 2022 at 21:49
  • Thanks for that context, makes sense.
    – A__
    Commented Jun 11, 2022 at 17:05

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