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I have a map with a "strange" projection (it's basically hand-painted, not an exact one). I can easily georeference it in QGIS using the Georeferencer plugin and the Thin Plate Spline transformation with a set of some 2000-3000 control points into simple WGS 84 (EPSG:4326) "latlong" projection and process/enhance it further with other data, like from Natural Earth or OpenStreetMap.

Now I'd like to perform the inverse operation: Transform one of those raster layers (or rasterised vector layers) back into the hand-painted quasi-projection, ideally using the same set of control points, so that the images match when overlaid in a graphic editor. How do I do it?

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interesting situation. Maybe you don't georeference the hand-painted map at all. Use the hand-painted map as 'ground-truth' and get your control points based on the local coordinate system ( say, top-left corner of hand-painted map is 0,0 and each pixel is a unit ). Then 'georeference' your rasters to this local coordinate system. –  spatialthoughts Jun 1 '12 at 12:20
    
@spatialthoughts: This would work for rasters rather well, however I would also lose the precision of vector layers and the possibility to do calculations with them (area of a feature, length of a segment and so on). That's not an insurmountable problem, of course. Being able to reverse the georeferencing just wouldn't have those limitations. –  Martin Sojka Jun 1 '12 at 13:20

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