My background: experienced programmer, amateur historian, complete beginner to GIS (so I'm using QGIS). I did learn to use the QGIS georeferencer tool though.
The input: suppose I have an image like this Wikipedia: File:VoyagesOfRabbanBarSauma.jpg. Assume that it's a "naked" JPG file without any geographic metadata and the author cannot provide any such information. The input image is licensed under GNU FDL, which I'm happy to release my derivative works under.
The goal: import the path shown in the Wikipedia map into QGIS so I can change the projection, superimpose elevation, add labels for physical geography, etc.
The caveat: one workaround might just be to enter the lat/long of the locations in the map into a new GIS project. I don't want to go this route because I'd like to have a record of that particular author's estimate of the geographic information.
What I've tried: I have applied the QGIS Georeferencer to the raster JPG file with a bunch of points around the continents' edges. The alignment of points in the interior of the continent are so-so.
What I'd like to know: the distortions of this particular map are pretty noticeable around the northern edge of Eurasia, and it seems like it's a pretty standard projection. It seems to me that there ought to be an algorithmic way to identify the projection and projection parameters that generated this map, and recover an accurate non-interpolation conversion between image pixels and lat/long?
I see two questions on this:
- Best practices for identifying the unknown coordinate system of a Shapefile
- Unknown Coordinate System on old drawing
These seem to talk about a brute-force method involving GDAL or QGIS, and to a GIS-beginner, seem somewhat labor-intensive (basically georeference and fit them to known projections) but I'm willing to try them and see if I can automate them.
The instructions on both these GIS.SE are a bit higher than my experience allows, and any hints on how exactly to do this brute-force method would be very appreciated. E.g., question 2 above shows how to use gdaltransform to convert a list of coordinates into lat/long. Are the input coordinates x-y pixel locations?