Latitude and longitude are the most common identifiers that point to a location on the earth. They also constitute the most precise and essential information for finding any location on a map. Many small communities and locales share the same name—a common problem in the region under consideration—but each has its own latitude and longitude, owning to its specific position on the Earth.
In mathematical terms, latitude and longitude are angular distances, with latitude being the distance north or south of the equator, while longitude is measured between the plane of a meridian through any point and the plane of a prime meridian or any selected datum meridian. Their numerical properties remain true for any sphere, including the Earth, and we retain that nomenclature when a sphere is flattened, as on a map. Thus every point on a map has a unique latitude and longitude.
The system of converging lines across a globe or map is known as a grid, essentially a referencing system by which points are noted by their distances from both a horizontal and vertical axis. It represents a branch of geometry devised by René Descartes, the French mathematician, natural scientist, and metaphysician of the seventeenth century.
Early Greek observations of latitude were employed by the Alexandrian cosmographer and geographer, Claudius Ptolemy, in his treatises on astronomy and geography in the mid second century A.D. Romans, Arabs, western Europeans, and Chinese all employed innovative features of designating parallels of latitude on maps and charts. Because the earth rotates on an axis, determination of longitude proved to be more problematic until the late eighteenth century, with the introduction of precise chronometers enabling cartographers to fix longitudinal distances. We go into a bit more depth on both concepts in the next section.
For our purposes, a grid is a network of lines over the face of map that represents meridians and parallels. We refer to the intersection of each meridian and parallel as a geographic reference point. This system of representing coordinates on a graph can be applied to a planar surface, such as a map. The virtue of planar rectangular coordinate system is that it enables us to fix any point on a map with great precision, which is necessary when searching for a place or feature name.