Gerard Mercator published his Nova et Aucta Aris Terrae Descriptes ad usum Navigatium emendate accomodata (A New and More Complete representation of the terrestrial globe properly adapted for its use in navigation) in 1569.
     The "properly adapted" refers to the particular quality of the Mercator projection on which loxodroms (rhumb lines--lines of constant compass bearing), as well as meridians and parallels are straight lines, which makes navigation by compass heading much simpler.