coordinate system by means of which the position or location of any place on the Earth's surface can be determined and described.

Latitude is a measurement on a globe or map of location north or south of the Equator. Technically there are different kinds of latitude—geographic, astronomical, and geocentric—but there are only minor differences between them. In most common references, geographic latitude (the kind used in mapping) is implied. Given in degrees, minutes, and seconds, geographic latitude is the arc subtended by an angle at the centre of the Earth and measured in a north-south plane poleward from the Equator. Thus, a point at 30°15′20″ N subtends an angle of 30°15′20″at the centre of the globe; similarly, the arc between the Equator and either geographic pole is 90° (one-fourth the circumference of the Earth, or ¹/₄ × 360°), and thus the greatest possible latitudes are 90° N and 90° S. As aids to indicate different latitudinal positions on maps or globes, equidistant circles are plotted and drawn parallel to the Equator and each other; they are known as parallels (parallel), or parallels of latitude.

Longitude is a measurement of location east or west of the prime meridian at Greenwich (Greenwich meridian), the specially designated imaginary north-south line that passes through both geographic poles and Greenwich, London. Measured also in degrees, minutes, and seconds, longitude is the amount of arc created by drawing first a line from the centre of the Earth to the intersection of the Equator and the prime meridian and then another line from the centre of the Earth to any point elsewhere on the Equator. Longitude is measured 180° both east and west of the prime meridian. As aids to locate longitudinal positions on a globe or map, meridians are plotted and drawn from pole to pole where they meet. The distance per degree of longitude at the Equator is about 111.32 km (69.18 miles) and at the poles, 0.

The combination of meridians of longitude and parallels of latitude establishes a framework or grid by means of which exact positions can be determined in reference to the prime meridian and the Equator: a point described as 40° N, 30° W, for example, is located 40° of arc north of the Equator and 30° of arc west of the Greenwich meridian.