In our extant literature from the Greco-Roman world, Claudius Ptolemy’s Geography is a unique treatise on mathematical cartography. Its relevance for Pelagios is in part that it includes the largest scientific data set to survive from antiquity: more than 6000 named points are located with longitude-latitude coordinates in books 2-7, and more than 350 key cities are again located in an alternate astronomical coordinate system in book 8. The text of book 1 provides a systematic introduction, including two methods for drawing world maps. Books 7 and 8 include instructions for other kinds of visualizations.
With its combination of methods and data, Ptolemy’s text is a kind of software for GIS. Ptolemy includes specific suggestions about how to realize his plan in hardware: he goes beyond the mathematical theory to discuss machinery and physical procedures for constructing maps. Scribes manually copying the text of the Geography applied these procedures directly to create maps in their manuscripts, but print editions have obscured the nature of Ptolemy’s project as a dynamic (if not rapid) GIS.
To understand Ptolemy’s work today, his software should be implemented in software. I have recently completed an initial digital edition of the Geography that is currently accessible via the Canonical Text Services protocol from http://ptolemymachine.appspot.com. From this edition, I can automatically extract all the named points with their lon-lat coordinates and several other attributes that Ptolemy assigns them in his text (e.g., type of feature, political unit, larger physical aggregate it belongs to…), and can serialize this information in a variety of formats for use in a GIS, including KML for direct use in Google Earth or Google Maps. (Above, Ptolemy’s points in Google Earth colored by Roman province or foreign “satrapy.”)
I am currently applying a variety of automated tests to the source text. In addition to traditional Greek “spell checking,” I am analyzing Ptolemy’s data for the spatial consistency of his attributes. (When all but one of the points assigned to a given Roman province fall uniquely within the convex hull of that group, but one point falls within the cluster of a different province, the most probable cause is a textual error. I’ll blog a couple of telling examples in a later post.)
During the period of the Pelagios project, I’m planning to complement the text-view of the Geography at ptolemymachine.appspot.com with a GIS view of Ptolemy’s text exposed through one or more network services. (I am still experimenting with a variety of options; I’m a newcomer to Google’s Fusion Tables, but am impressed with their easy access to people who want to mash up geographic features, in the spirit of Pelagios.) I’m looking foward to seeing what others might do with this material.
College of the Holy Cross