In common parlance, means something simple and straightforward, but the origin of the term is purely maritime. When the chart first appeared in Italy towards the end of the 13th century, it was drawn as plans are drawn, with no reference to the sphericity of the earth. The observed bearings and measured distances were plotted irrespective of the principles of celestial navigation and the convergence of the meridians towards the poles. That the earth was a sphere was, of course, understood and taught in the universities. But the limited coverage of charts within the Mediterranean made any relationship to the spherical surface of the earth unnecessary. However, the expansion of exploration by sea beyond the Mediterranean, and the practice of running down (i.e. along) an astronomically observed latitude, made it impossible to ignore the shape of the earth. Nevertheless, seamen continued their practice of drawing the sketch maps in the sensible and uncomplicated way they were used to and let those involved in chartmaking cope as best they could.
In 1599 Edward Wright published his Certaine Errors in Navigation which contained a table of meridional parts which gave the spacing of the minutes of latitude along the meridians. This enabled the ordinary chartmaker to produce a chart on Mercator's projection with parallel meridians on which the mariner could still draw a rhumb as a straight line. The chart became known as the true chart, as opposed to the ordinary, or simple, or plain chart, though one school of thought holds that the word ‘plain’ is simply a corruption of ‘playne’, meaning a flat surface. The references below give the arguments for either opinion. Whichever is right there can be no doubt that the introduction of the true chart was one of the most important advances in the practice of navigation.
Taylor, E., ‘All Plain Sailing’, Journal of Navigation, 9 (1956), 230.Waters, D., ‘Plain Sailing or Horizontal Navigation’, Journal of Navigation, 9 (1956), 454.
Subjects: Maritime History.