Maps that use the geographic system use the latitude and longitude designations. Latitudes use the equator as 0 and go north and south. Longitudes use the Greenwich Meridian, or Prime Meridian, as 0 and go east and west.
The equator is designated as 0 degrees and has poles at 90 degrees. The latitudes are equally spaced north and south.
Longitudinal lines use the North and South poles as ends. Greenwich, England is 0. From the Greenwich Meridian, the lines are 10-degree increments around the globe in the east-west direction.
Military maps base their grids on the XY coordinate system. The lower-left corner is 0, 0 and increases to the right and vertically. Each square on the map represents 1,000 X 1,000 m.
Countries may make maps not based on the Greenwich Meridian, and use their own location. Maps from these countries designate the difference between the Prime Meridian and the one they designated.
Grids make a map easier to use. Road maps have grids and an index. Look up a location in the index, and it gives a grid where to look.
