So since the USGS is releasing a bunch of their old scanned paper maps at their Historical Topographic Map Collection, I thought it would be interesting to take the GeoPDF's there and georeference them to GeoTIFF's to compare to modern maps. I've always had a thing about history, so thought this would be a fun side project to get my cartography back on.
Since QGIS doesn't yet import GeoPDF's, I'm first loading them into the GIMP (at around 300 dpi from the PDF), clipping the collar out, and saving them to a LZW-compressed TIFF. Then I'm importing the TIFFs into QGIS and using the Georeferencing plugin to mark the grid points on the map. I've read that the old maps were based on the Clarke 1866 ellipsoid, so in QGIS I'm setting the source projection to NAD27, which is based on Clarke 1866. Yes, I know that technically this isn't fully correct in the cartographic sense, but then again georeferencing old paper maps like this also won't exactly put out a highly accurate GIS product either :P I'm outputting them to WGS84 from the georeferencing plugin. Times on my older Dell E1705 are around 5 minutes doing a polynomial interpolation with Lanczos interpolation. Then again, I remember back in the mid to late 1990's when DRG production at the USGS on the old Data Generals would take a whole lot longer, so I wasn't going to complain :)
The output isn't so bad really. Here's a sample of the output draped on top of Yahoo Street Maps (Google Maps and reprojection on the fly doesn't seem to work so well in QGIS right now).
Setting the Fredericksburg 1889 map to 50 percent transparency in QGIS and zooming in to old town, you can see that the map fits to a modern map surprisingly well.
I'll probably play around with this some more and maybe upload the georeferenced maps to archive.org or something along those lines.