The purpose of a map is to express graphically the relations of points and features on the earths surface to each other These are determined by distance and direction In early times distance wha often expressed in units of time for example so many hours march or a days journey by river but such measurements gave more information about the relative ease of crossing the local terrains than they did about actual distance The other element is direction but for the ordinary traveller whose main concern was Where do I go from here and How far away is it the accurate representation of direction was not of primary importance Partly for this reason written itineraries for a long time rivalled maps Even today certain types of maps for example those showing railway systems may make little attempt to show true directions Similarly conspicuous landmarks along a route were at first indicated by signs realistic or conventional and varied in size to indicate their importance Clearly the conventions employed varied with the purpose of the map and also from place to place so that in studying early maps the first essential is to understand the particular convention employed