Collecting the locations of places
In the second century CE, Ptolemy worked in Alexandria on a Geography that organized cities, rivers, mountains and coasts for mapping. He drew on predecessors such as Marinus and on reports of travel and navigation. Places received latitude and longitude and were arranged by region. A reader could locate a name in the lists and transfer its numerical position to a map, giving information gathered across distant journeys a shared spatial framework.
Drawing a sphere on a flat surface
The theoretical chapters discuss the Earth as a sphere, its coordinate grid and methods of projection. A mapmaker had to choose how to place curved parallels and converging meridians on a flat surface. Instructions connected numbers to a reproducible drawing. Astronomical observation could help establish latitude, while longitude depended more heavily on routes and estimates. Differences in information and distance estimates produced uneven errors in the shapes and extent of mapped regions.
Coordinates through manuscript and print
Byzantine manuscripts preserved the text and its cartographic tradition. Latin translations and printed editions brought it into European scholarship in the fifteenth century; the Rome edition of 1478 included engraved maps. Mapmakers later compared coordinates with new voyages, changed coastlines and added regions absent from the ancient work. Numerical lists could be recalculated and redrawn, allowing names, locations and geographical outlines to be revised through successive copies and editions.