What I have: A large TIFF covering parts of several contiguous sheets of the British OS 6 inch first edition. It has no georeferencing. It's just a series of scans which someone has stitched together (actually gdal_transform extracted it from a larger .ecw)
A number of scans of original sheets. These are also plain scans with no georeferencing but they do have latitudes & longtitudes along the borders so I can read off the corner coordinates in DMS.
I assume that the projection of all this data will be close to if not identical to the current OS projection.
What I want to achieve: The large TIFF properly georeferenced so it can be used as a basis for georeferencing various tithe maps.
How I expected to go about it: Set the existing CRS of one of the scanned original sheets to OSGB70 which, as far as I can see is the OS projection using lat/long, fire up the georeferencer, set GCPs at each corner, set the output to OSGB70 & run the transform. Repeat for other sheets. Using these georeferenced sheets pick off GCPs for features on the big map to use the georeferencer to georeference this.
What happened: A grossly distorted map was produced. I think I can see what the problem is; the original is in the OS projection but its CRS is purely pixels & the georeferencer treated it as degrees. But what do I set as the initial CRS to set up the correct transform?