One of the earliest maps of the world is traditionally attributed to Anaximander of Miletus (610-547 BCE) who, in the fifth century BCE, had imagined the inhabited lands occupying a circular space around the Aegean Sea. However, the basic principles of geographic representation were established by Eratosthenes of Cyrene (ca. 276-ca. 195 BCE). After having accurately measured the Earth's circumference, Eratosthenes was the first to include the grid of meridians and parallels in the geographic representation. His map incorporated the information gathered during the military expeditions of Alexander the Great (356-323 BCE). The later measurements by Posidonius (ca. 135-middle of 1st cent. BCE) helped to consolidate the geometrical model of the terrestrial globe, establishing geometry and astronomy as the foundations of geography. On this basis and using astronomical observation, Hipparchus of Nicea (2nd cent. BCE) established the method for calculating the longitudes of localities. Hipparchus also speculated about the existence of a landmass between the Indian and Atlantic Oceans, a hypothesis based on the observation of the difference in tide patterns in the Arabian Sea and on the Atlantic coasts. The information was reported by Strabo (before 60 BCE-ca. 20 CE), whose Geography in 17 books is an essential source of knowledge about the ancient world. His narrative description of the history of peoples and the distribution of landmasses served as a reference for geographers such as Pomponius Mela (1st cent. CE), the only Latin author to write a comprehensive geographic treatise. Pomponius divided the Earth into five climate zones, of which he claimed only two were habitable. He borrowed from Eratosthenes the description of the boundaries between Europe, Asia, and Africa. Like all classical geographers, he regarded the Caspian Sea as a large gulf connected to the Northern Ocean. One of the precursors of Ptolemy's great geographic compilation was the work of Marinus of Tyre (2nd half of 1s cent.-1st half of 2nd cent. CE), who was the first to describe China as the eastern boundary of the known world. The inhabited world, according to Marinus, extended in longitude from the Fortunate Islands—today's Canary Islands—to China, and in latitude from Thule, probably the Shetland Islands, to the Tropic of Cancer. Marinus coined the term "Antarctic," understood as the pole opposite the Arctic, and he was the first to assign geographic coordinates to every individual location. Longitudinal coordinates were numbered from the zero meridian, which passed through the Fortunate Islands. Marinus drew a world map based on a regular orthogonal grid with equally divided intervals between meridians and parallels; he described the longitudinal extension of the oikumene (inhabited world) as equal to 225° of the Earth's circumference. Every meridian degree was 500 stadia, or 185 meters, long—as against the 700 stadia set by Eratosthenes. Ptolemy (ca. 100-ca. 175 CE) incorporated a large portion of Marinus's geographic data but, for the representation of landmasses, he accepted the orthogonal division of meridians and parallels only for regional maps. For the general map of the oikumene, he developed personal methods for transforming the Earth's sphere into a flat surface.