were further classifications. Divisions between continents and nations were often determined by bodies of water. Greek maps further separated the world into habitable and inhospitable zones, and some maps included the concept of the antipodes, an oikoumene in another habitable zone of the Earth not accessible with Greek transportation technologies. Later Greek scientists determined that the Earth was round, although there was some debate as to the accuracy of this claim. During the conquests by Alexander the Great, Hellenistic knowledge of the world was expanded, resulting in an improved map of the world by Eratosthenes. He also calculated the circumference of the Earth, and published two treatises about geography, titled Geographica and On the Measurement of the Earth. Contemporaries of Eratosthenes questioned the validity of his methods, although his calculation of the Earth's circumference was quite accurate. Roman maps Like the Greeks, the Romans did not possess the materials or technology to produce useful maps. Several examples of what may have been Roman maps have been introduced through archaeology, although both their validity as maps and even their authenticity have been debated. Romans used itineraria in warfare, to guide government postal workers, and for civilian travel. Those that contained illustrations to underscore the text would not have been to scale, and would have born more similarity to transit maps than contemporary to-scale maps. Although not used for travel, there is evidence of maps created after surveys by the Roman government, in a process known as centuriation in which the land was reduced to grids for cultivation. Travel narratives Writings in Greek that could be qualified as travel narratives included Histories by Herodotus. Sections of the work are devoted to geographic and ethnographic descriptions of the territories encountered during his travels, which ranged from Egypt to the Persian Empire. Strabo also compiled his travels into a major work, Geographica, which outlined his conceptions of the world and the geographies of regions such as what are today Spain and India. Narratives from Rome that include travel include Caesar's Commentarii de Bello Gallico, which describes Caesar's conquest of Gaul. Tacitus' Germania relies on information he gained from those who had journeyed to Germany, although he never did. Paradoxography Paradoxography is a genre of classical travel writing that recounts encounters with foreign or supernatural peoples, animals, and events. There were whole works devoted to such descriptions such as works by Palaephatus, and an otherwise