<P> Grid references define locations on maps using Cartesian coordinates . Grid lines on maps define the coordinate system, and are numbered to provide a unique reference to features . This reference is normally based on projected easting and northings </P> <P> Although professional map - making and use of the grid had existed in China before, the Chinese cartographer and geographer Pei Xiu of the Three Kingdoms period was the first to mention a plotted geometrical grid reference and graduated scale displayed on the surface of maps to gain greater accuracy in the estimated distance between different locations . Historian Howard Nelson asserts that there is ample written evidence that Pei Xiu derived the idea of the grid reference from the map of Zhang Heng (78--139 CE), a polymath inventor and statesman of the Eastern Han dynasty . The American writer Robert K.G. Temple asserts that Zhang Heng should also be credited as the first to establish the mathematical grid in cartography, as evidenced by his work in maps, the titles of his lost books, and the hint given in the Book of Later Han (i.e. Zhang Heng "cast a network of coordinates about heaven and earth, and reckoned on the basis of it"). </P> <P> Grid systems vary, but the most common is a square grid with grid lines intersecting each other at right angles and numbered sequentially from the origin at the bottom left of the map . The grid numbers on the east - west (horizontal) axis are called Eastings, and the grid numbers on the north - south (vertical) axis are called Northings . </P> <P> Numerical grid references consist of an even number of digits . Eastings are written before Northings . Thus in a 6 digit grid reference 123456, the Easting component is 123 and the Northing component is 456 . </P>

Grid lines that are drawn in a north-south direction