 The rule of marteloio has sailors plot routes by drawing plane triangles on a chart, as if the world's surface were flat.While this might be practical enough for sailing confined to the compact latitudes of the Mediterranean Sea, it is quite misleading on a grander scale.In the late 15th and 16th centuries, the improvement of nautical astronomy and the introduction of latitude parallels allowed navigators to determine their position at sea by celestial readings, rather than relying on estimation of distance sailed.