For most persons, the words 'map' and 'chart' are freely interchangeable. However, a distinction may be made between the two. Concern of landsmen is with the geographic characteristics within their realm--its rivers, mountains, forests and swamps, along with man-made features of roads and towns--in order to grasp the nature and extent of the land and know how to move around within it. Maps provide that information. Oceans are considered merely as blank spaces separating the land-masses. The mariners' interest is the reverse; their primary domain is the ocean, and land is portrayed only to define the outer bounds of that region. Interest in land is confined to that thin strip containing harbors of refuge at the end of a voyage. Only those features of land visible from the sea, such as headlands and prominent features, which enable him to correctly identify a landfall, are of importance. Charts, then, are a "mapping" of the ocean.