Both longitudes as well as latitudes are essential in locating a place on a globe or a map, A place at 80°E can be anywhere along the 80°E meridian extending from pole to pole. But if we know that the latitude of a place is 30°N, then we can easily locate it as there can be but only one place at that point. For example looking into a map we know that the actual place having 80°E, 30°N position is Chennai.
Similarly if we know the name of a place and would like to know where it is situated we could take the help of an index. It is provided in every atlas or a map, which is an alphabetical listing of all the places on that map. It is with the help of the two symbols following it that we locate a place. For example, if we would like where Lucknow is, the index shows Lucknow 26.55 N 80.59 E. We have seen earlier that letter N and S follow latitudes while E and W follow longitudes. Thus we look into the 26,55° north latitude and the line which intersects it at 80.59° eastern longitude gives us the location of Lucknow. Lucknow is 26.55° north of 0° equator and 80.59° east of prime meridian.