How did Ptolemy use latitude and longitude in his Guide to Geography?

In his Guide to Geography, Ptolemy revolutionized cartography by recording longitudes and latitudes in degrees for approximately 8,000 locations. This systematic approach allowed for the creation of an exact duplicate of his world map, providing a detailed representation of the world as understood during the height of the Roman Empire.12

Beyond simply listing coordinates, Ptolemy devised two methods for projecting a grid of lines onto a flat surface to represent the Earth's spherical latitude and longitude on a map. His more advanced projection used circular arcs for both parallels and meridians, which helped preserve the proportionality of distances and anticipated later cartographic techniques. Although his maps contained distortions due to the limited accuracy of the trade and road data available to him, his work became a foundational influence for Renaissance cartographers.21