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I am currently building a global database of sutures, major faults and thrusts, orogenic events, magmatic episodes and plate boundaries with the goal of building a framework of Earth's tectonic evolution over the past three billion years.

I am surfing the internet for maps to digitize and add to my database. Unfortunately, I have been mostly roadblocked for the past several months because most of the maps I am using as reference materials are in the wrong map projection. GPlates, the software I am using for this project, only accepts rasters that are in simple equirectangular, or latlong, projection. But most of the maps that I have found are in other projections in order to minimize distortion.

I emailed a geophysics researcher about this problem and showed him several of the problematic maps, asking him for help in georeferencing them. He said that he unfortunately didn't have the time to undertake that task for me, but he did recommend QGIS to me as a software that was able to georeference the images.

I downloaded QGIS 3.12 and experimented with it for a while to try and accomplish a correct reprojection of a few of the images into simple latlong, but despite watching several QGIS tutorials and reading a large amount of documentation in online user manuals and sites like this one, I was never able to produce any georeferenced maps without unacceptable amounts of error.

None of these maps contain any information whatsoever about what projection they are set in. Most of them don't have a grid, only some latitude and longitude tick marks along their edges. Some of them have no latitude or longitude information at all, and show only coastlines and political boundaries for reference. I couldn't find any helpful information online about how to figure out what projection a map is in when that information is not given. I feel like I am trying to squeeze blood from a stone with this project.

I am clueless, and absolute beginner, and need help. For reference, I have attached one of the problematic images that is set in an unknown map projection.

This one doesn't seem to be in a conic projection, because I used the manual raster transform plugin and couldn't get it to align with any such projections. But I otherwise know nothing about its projection.

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Georeferencing without prior knowledge of the coordinate system is difficult task, and my usual strategy largely depends on trial-and-error.

To me the map looks either on Lambert Azimuthal Equal Area (LAEA) or Lambert Conformal Conic (LCC); let me start assuming LCC (EPSG: 3034). (There may be more appropriate one).

(Step 1) Find a world map which we can use as reference.

One good data source is Natural Earth dataset. Choose any one of worldwide coastline from physical vector dataset.

(Step 2) Load coastline data to QGIS and change the project projection to EPSG: 3034.

(Step 3) Install Freehand raster georeferencer plugin. Add and adjust location of your raster image using this plugin, then export it as a Raster image with world file. (by the green icon with red !! marks).

enter image description here

(Step 4) Reload the exported raster to QGIS project again, and Export (save) it onto WGS84 Latlong (EPSG:4326).

[Edit]

To check the coordinate system and/or adjust your image more precisely to the basemap, you could add a graticule layer by Create grid tool (in Processing toolbox > Vector creation).

enter image description here

enter image description here

My bad, my attempt was not so good...

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  • Is there a graticule plugin you could add to the basemap in order to confirm the lat-long lines at the tickmarks?
    – Spacedman
    Commented Apr 5, 2020 at 9:32
  • @Spacedman Nice idea! I honestly don't know of any, but will give it a try with Create grid tool.
    – Kazuhito
    Commented Apr 5, 2020 at 9:41
  • Is there a way to anchor points on the raster to fixed coordinates, so that those points stayed still while the raster was scaled or rotated? Because I feel it would be much easier to georeference a map if one or two points on the raster were held fixed during manual transform.
    – Samuel Philippian
    Commented Apr 5, 2020 at 18:23
  • @SamuelPhilippian That would be nice feature, but I do not think this tool has that ability.
    – Kazuhito
    Commented Apr 5, 2020 at 22:20

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