This is a system of imaginary lines superimposed on the earth to aid in locating points. Primitive man probably developed these concepts in relation to the direction of the rising and setting sun (and the time required for travel). All spatial locations are relative and must be established in relation to some starting point. Once that point has been established, the location of other points can be stated in terms of a defined direction and distance from it.
The Greeks devised a system of locating a point between the two poles. They imagined a series of east-west circles around the earth parallel to one another. The one dividing the earth equidistant between the poles was appropriately called the equator. These parallels were latitudes and have persisted for some 2,200 years establishing north-south positions. The need for an east-west coordinate was recognized early on. Ptolemy in 150 A.D. used the Babylonian 360-degree circle to divide the world into zones.
Locating the prime meridian was a different, more difficult problem. In 1493 Pope Alexander VI issued the Bull of Demarcation establishing a meridian (extending from the North to the South Pole) 100 leagues west of the Azores. However, no one really knew its precise location.
Prime meridian was a continual source of controversy between Spain and Portugal, Europe’s foremost maritime rivals. Each country independently established its own capital (Madrid for Spain; Lisbon for Portugal) as the prime meridian (zero-degree longitude) and published their own maps and charts to reckon from these points. No international standard existed until 1884 when a world conference agreed on Greenwich, England, as the location of the prime meridian. This is almost universally accepted.
Grids on maps help us immediately locate, by latitude and longitude, and approximate the relative sizes of places shown.