Some of the barriers that existed between me and the realization of this map could have been resolved by a few thousand hours of work on my part. I could have hand-located villages that do not match up with post-Revolutionary communes.
I was lucky enough at the time to receive good advice about the challenges of fitting 21st century GIS geodata into an 18th c. box. While many places could be reliably located using official French data (provided by the IGN), just as many places whose names have changed or whose jurisdictions have grown or disappeared would not match current administrative geography. IGN data contains some information on alternative or historical spellings and defunct communes, but this information only goes back as far as 1790.
Accounting for pre-Revolutionary places would mean drawing on location information from other sources. Like populated places, eighteenth-century roads do not cleanly match up to roads today. GIS datasets for French transportation infrastructure (such as THIS) represent the continual re-configuration of roads over time making it impossible to identify “eighteenth-century” roads. Again, locating eighteenth-century roads would need to be accomplished by referring to contemporary sources like the Cassini map sheets of the mid to late eighteenth century. The Cassini sheets for Brittany were surveyed and printed at the very end of the Old Regime (in 1789 and into the 1790s). They are a snapshot of the road network at its most “completed” point before the renewal of interest in road building later in the nineteenth century. (As of 2011, however, features of the Cassini maps had not been digitized.) Documenting Brittany’s road construction environments would mean capturing a few of these snapshots from printed and manuscript map sources, but
drawn approximations of roads between them based on the evidence from contemporary manuscript maps. But this effort would have been ultimately full of errors