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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.

PolyGeo
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Alexander Huber
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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 '21 at 19:34
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    But you do know that you could simply export the already georeferenced version? – MrXsquared May 24 '21 at 19:48
  • Those four or five "georeferenced" versions would be mine, none of them work although the georeferencing "appears" to be perfect. :-( – Alexander Huber May 24 '21 at 19:53
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    Just tried this one and its looking just fine https://exports.georeferencer.com/f144aae0-512d-5991-bffc-1114b1473551-2021-04-11T21:47:05.995085Z.tif (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 '21 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? – Alexander Huber May 24 '21 at 20:18
  • @MrXsquared your link worked fine, no login needed. Georeferenced map is placed correctly for Eurpa, Africa, Asia and Australia, but not for the Americas: https://i.stack.imgur.com/TNX1o.jpg - nor does changing project EPSG to a pacific-centered one solve the problem, still only the same extent as before is shown correctly. – Babel May 24 '21 at 20:38

1 Answers1

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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

Babel
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  • Thank you so much for your work with this, I just tried the georeferencing, but after a few points I got a huge mean error. When I removed the offending point, I got: "Transform Failed: Failed to compute GCP transform: Transform is not solvable." I'll try with exactly the points you chose next! – Alexander Huber May 24 '21 at 20:25
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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 '21 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 '21 at 20:32
  • Ha, still no luck. After setting control points, georeferencing is stuck at 4% progress. You have made so much more progress in a short time. Just as I'm aware I'm taking up too much of your time: are you aware of any "professional" services that would produce a perfectly-looking georeferenced version of a map like this? I wonder what I'd have to budget for a service like this? Thanks for any advice you may have! – Alexander Huber May 25 '21 at 06:29
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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 '21 at 06:54
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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 '21 at 06:56
  • Thanks, again! So, I can set the first four control points. But the one near the Bhering Strait is already on the left hand side of the OSM map I use and as soon as I set it I get a huge error. The same happens when I set the point near Cape Horn. (not sure how to add screenshots to demonstrate?) – Alexander Huber May 25 '21 at 07:13
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    How does your OSM basemap look like? Is it Pacific centered as can be seen here: https://i.stack.imgur.com/pZBfw.png If not, set project CRS (at the bottom right). If you edit your original question, you can paste screenshots - the same way you added the map image when you originally asked the question. – Babel May 25 '21 at 07:19
  • Oh facepalm, sorry yes that would explain things, trying it now with the correct CRS! Thank you! – Alexander Huber May 25 '21 at 07:25
  • OK, great, yes I've done that and it looks good. When I change the CRS to 3857 it puts the map together like you showed in your screenshot. Sorry, more questions: 1. the control points aren't aligned when I change to 3857, is that just a visual error or should they be in the correct position (see snapshot)? 2. ultimately I'd like to use Openlayers or Leaflet to display the map, am I right in thinking that I need to use 3857 and export it as a GeoTIFF so this will look correct? 3. is there a way to display the map as it was originally (so pacific-centred), but still present it correctly online? – Alexander Huber May 25 '21 at 07:48
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  • Not sure exactly what you mean an if that's a problem, 2.) probably yes, 3857 is the way to go, but I'm not a Openlayers/Leaflet expert and not 100% sure if it allows other projections as well; 3. Did you have a look at QGIS cloud? It's a good way to serve web-maps based on a local QGIS project: https://qgiscloud.com/
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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! – Alexander Huber May 25 '21 at 08:20
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    When I read the question, I supposed that what I was to propose could work, but I was not sure. So I tried it and see: it works. So I learnt something, too! – Babel May 25 '21 at 14:56
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    If you have a webmap to share (now or later), it would be nice to see the result – Babel May 25 '21 at 18:39
  • Just added a screenshot. Thanks again very much! – Alexander Huber May 25 '21 at 21:47