Background I'm starting with a map from the internet (Encyclopaedia Britannica) and georeferencing it using QGIS 2.0.1, I get really accurate georeference solution using polynomial 2 transformation: very small residuals, most less than 5 pixels. (This is surprising because the previous time I tried this, I got pretty poor results: Improving Georeferencing results? though now I am using more points (31) and was quite careful with them.)
The georeference window:
(note the very slight residuals, the biggest of which is the small line emanating from Beijing). This produces the following overlay, which you can tell is quite good.
Goal I'd like to identify the projection and its parameters used by the cartographer.
Questions Can the fact that a polynomial 2 fits really well be used to narrow down the list of projections that the map uses? Or do many/most projections georeference well with polynomial 2 when applied over such a large geographic extent and with so many control points?
(I can disable most of the control points and as long as I include at least one in the northern regions, I can get the same very good georeference. And of course polynomial 3 works very well also.)
(I've also tried to fit most of the pseudocylindrical projections mentioned in The Proj Manual (1996, revised 2003) to see if one can be made to match the control points as well as the polynomial 2 georeference fit, but the latter is much better than any of those fitted projections.)
The GCPs I have put the GCPs at http://pastebin.com/pWti0W8k. The map x/y are in EPSG:4326 (I used the Google Physical OpenLayers map as the reference but then reprojected those EPSG:3857 to EPSG:4326 so I could get lon/lat).