The time period covered is attributable both to the material itself and existing work done in the County. The first printed maps for the County appear in Elizabethan times, with topographical prints appearing slightly later, which sets the start date for the project. By the late nineteenth century, printing technology is changing rapidly leading to new, more diverse book illustrations, and of course photography transforms the process of recording images of places. This transformation makes it much more difficult to maintain a distinction between published and unpublished items, as well as keeping track of "editions" of an image, in the way that was more reliable in the days when an image was stored by cutting it into a metal plate.
The publication of the large scale maps of the County by the Ordnance Survey in the 1850s marks a distinct cut-off point for maps. From this time, accurate maps would in the main be based upon this data rather than being surveyed on the ground (or more or less plagiarised from predecessors' work).
The dates chosen also ensured that the prints were out of copyright and could be scanned and placed on the WWW. Finally, the period dovetailed neatly into a project already completed by Durham County Council's Arts, Libraries and Museums Department, The Durham Record, which digitised large scale Ordnance Survey maps and photographs relating to County Durham.