Georeferencing historical maps is a time consuming process as historical maps are matched to a contemporary map with spatial coordinates by clicking on a point in the image that matches a point on the map. The more points you locate the more accurate the image will align with the base map. The process is a key element to all historical GIS projects.
In the Stepping Into Time Project we will be georeferening historical maps of bomb locations. There are a number of crowd sourcing georeferencing projects from a number of the larger archives including this great one from the British Library. In the British Library interface you can select from a wide variety of base layers to use as your mapping base including: OpenStreetMap, Ordnance Survey Open Data UK, Google Street View/Hybrid and Terrain.
It is really fun and I have just tried to georeference a map of Abergavenny from 1813. It is not that easy because the place names have changed (and are in Welsh). To complete the process I used a variety of the base layers available. Including Google Terrain to help locate which valley I was in, Ordnance Survey Open Data and OpenStreetMap to locate the actual villages or farms to act as control points. The historical map marks all the old mines (which is fascinating) but these are not marked on the modern base maps so it is a little tricky to align despite knowing the area. What is nice is you can review the map you have just georeferenced in Google Earth which is embedded into the British Library webpage. I need to go back and improve the accuracy of my work.
It is quite a fun thing to do and is slightly addictive – like doing a jigsaw puzzle!