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I've unsuccessfully tried to georeference this world map.

World map

Whatever I try, however many control points I set, I get garbage when I view the resulting GeoTIFF. I've tried https://www.georeferencer.com/ (in which I've set 60 control points with minimal deviation) and QGIS 3 (which has yielded nothing but "space-warped art") and I have simply no luck.

Is there an issue with the map anyone could identify?

Map overview: https://www.davidrumsey.com/luna/servlet/detail/RUMSEY~8~1~2722~260002:The-World,-on-Mercators-Projection-

Download link: https://www.davidrumsey.com/rumsey/download.pl?image=/D0026/4613002.sid

enter image description here

Bottom control point is Cape Town, is that just a visual error or a real one?

enter image description here

Finished product, there's actually a thin blue line where the base map shines through in the middle that I don't know how to get rid of, but other than that it looks great.

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    I would have a try by splitting the map at the meridian of 180 degrees and georeferencing the halves separately.
    – user30184
    May 24, 2021 at 19:34
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    But you do know that you could simply export the already georeferenced version?
    – MrXsquared
    May 24, 2021 at 19:48
  • Those four or five "georeferenced" versions would be mine, none of them work although the georeferencing "appears" to be perfect. :-( May 24, 2021 at 19:53
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    Just tried this one and its looking just fine exports.georeferencer.com/… (Maybe you need to login to get the link content). What do you mean with appears to be fine but none of them work?
    – MrXsquared
    May 24, 2021 at 19:55
  • Well, what I want to do is create a web version, so when I do: gdal2tiles.py --xyz f144aae0-512d-5991-bffc-1114b1473551-2021-04-11T21\ 47\ 05.995085Z.tif mymap and then look at /mymap/openlayer.html it is broken. So I'm clearly missing some in between step to achieve that, or the georeferencing didn't work? May 24, 2021 at 20:18

1 Answer 1

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  1. Set your project CRS to a pacific centered CRS like EPSG:3832 or EPSG:8859. Your map canvas with basemap should than look like in screenshot 3 below.

  2. Than georeference your map using the same CRS for transformation settings (screenshot 2).

  3. Once you georeferenced the image, you can change your project CRS back to the one you used before or re-project your georeferenced image (screenshot 3).

You might want to split the image at the antimeridian (180th meridian) for better results. See: https://gis.stackexchange.com/a/396562/88814

Be aware: with this map, transformation settings work fine with projective transformation as the map is in mercator projection and you transform to another mercator-projection CRS (EPSG:3832). In other cases, you might to select other transformation settings like polynominal2, polynominal3 etc. See: https://gis.stackexchange.com/a/370248/88814

Screenshot 1: Setting project CRS to a Pacific-centered one (at the bottom left), your basemaps should look something like this: enter image description here

Screenshot 2: Control-points, Transformation settings and results; red lines is the pre-installed polygon layer with coastlines/country borders: enter image description here

Screenshot 3: result when changing the project CRS to 3857 with the image geo-referenced in EPSG:3832: the image is split automatically, but it doesn't look perfect:

enter image description here

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    Indeed - if you use the "wrong" control points, you get this kind of error messages (plus long, red lines indication errors). Try to avoid points that lay on the part of the old map that overlaps (like Great Britain appears twice on your map). Points east of the British islands should be fine, as well as west of the Azores. So points in central and eastern Europe and Africa, but especially Asia, the Pacific and the Americas should be fine.
    – Babel
    May 24, 2021 at 20:30
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    At least avoid the "right" part of your old map. If you set control points in the UK, Spain, Western Africa, than do it on the "lft" part of the map, where these regions appear again, and not on the right side.
    – Babel
    May 24, 2021 at 20:32
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    If you 1.) set project CRS to 3832, 2.) only take the same 6 control points I used to test (see my screenshot) and 3.) use the exactly same transformation settings as I used (see my screenshot), especially CRS again to 3832, than do you get an acceptable result? If not, can you post screenshots of 1.) Your map canvas with background basemap, 2.) control points you set, 3) transformation settings you use? Edit the question for this. And maybe also if possible make available the control points you use + the exact image file you use for for Geocoding.
    – Babel
    May 25, 2021 at 6:54
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    So before recurring to a payed option, let's see if you can do it yourself. Has the bonus of learning something and being able to repeat it in the future for other maps.
    – Babel
    May 25, 2021 at 6:56
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    Just tried it in Openlayers, almost perfect! Thank you so so much, you've been a fabulous coach and a life-saver, and thank you too for putting up with a noobie questions and my mistakes! May 25, 2021 at 8:20

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