One of the oldest systematic methods of location is based upon the geographic coordinate system. The distance of a point north or south of the equator is known as its latitude. The rings around the earth parallel to the equator are called parallels. A second set of rings around the globe passing through the poles is known as meridians of longitude. A meridian is designated as the PM. The four lines that enclose the body of the map are latitude and longitude lines.
The U.S. Army military grid reference system reduces the length of written coordinates by substituting single letters for several numbers. Based on the military principle for reading maps (RIGHT and UP), locations on the map can be determined by grid coordinates. Each of grid lines is labeled at both ends of the map with its false easting or false northing (distance north of the equator) value.