I'm wanting to georeference and vectorise some features from scans of the OS 1896 Town Plans that I have purchased from the NLS (http://maps.nls.uk/os/london-1890s/info.html). The scans provided are not georeferenced and when I try to crop the borders they are not quite square so will need rotating first. The problem is the features lie at the intersection of 4 separate sheets and I'm uncertain of the best / most accurate approach to take to merge them:
1) Treat each image separately: Rotate and crop each image in Gimp, georeference in QGIS, then "nudge" the individual GCP points to try get them to line up properly, then merge with gdal_merge?
2) Merge first: Rotate, crop and merge the images together in Gimp, then georeference the whole image in QGIS
3) Some magic automated solution I'm not aware of :-)