You’ve seen them on trail maps, road maps, and engineering plans. For centuries, contour lines have been a standard cartographic convention. Used to represent the elevation of land and mountains, to modern map-readers the curving marks are implicit representations of reality, as true as the blue of an oceanic map.
But as with most things in this human-built world, contour lines had to be invented. Their origins lie with Charles Hutton, a British mathematician whose ambitious 1774 survey of a Scottish peak called Schiehallion marked their first known use. That map was lost to history, but his original charts and tables of survey points were not.