Nautical chart always true north, rather than magnetic north, towards which a compass points. Most charts include a compass rose depicting the variation between magnetic and true north. However, the use of the Mercator projection is not without its drawbacks. Mercator's technique was to make the lines of longitude parallel. On the real globe, the lines of longitude converge as one goes north or south away from the equator, until they meet at the pole. This distortion means that horizontal distances are exaggerated. Mercator's solution, imperfect as it might be, was to increase the distance between lines of latitude in proportion; in
Mercator 1569 world map 12 by using compass roses on the equator and it is only in Legend 10 that he introduces the Organum Directorium and also addresses the inverse problems: given the initial point and the direction and distance of the second find the latitude and longitude of the second. Mercator's construction is simply an evaluation of the rhumb line distance in terms of the latitude difference and the azimuth as If the latitude difference is expressed in arc minutes then the distance is in nautical miles. In later life Mercator commented that the principles of his map had not been understood by