North and South, by definition, is a position along a line of latitude, and is measured in degrees, minutes and seconds, up to 90 degrees 0' 0". The parallel of latitude that corresponds to 0 degrees 0' 0" is the equator. Because of how maps and navigation work, if you are standing at the (true) North Pole, and decide to move in a given direction, you cannot be travelling north, as you would be moving towards the equator. East and West are a little different, because of the nature of meridians of longitude. Like lines of latitude, lines of longitude are also measured in degrees, minutes, and seconds, and are measured up to 180 degrees east or west from the Prime Meridian, which passes through Greenwich, England. Due to the nature of lines of longitude, you'll always be travelling east/west if you maintain an east/west heading, but if you were to travel far enough, you will not always be travelling north/south by maintaining your north/south heading once you pass over either pole.