The Invention of the Contour Line

[A] Before the contour line a map showed relief by drawing it. Hills were small mounds seen from the side, or bundles of short strokes laid down the slope, the darker the steeper. [B] Such maps are easy to read and useless to measure. Two hills drawn with the same shading might differ by a hundred metres, and nothing on the sheet would say so. [C] The contour gives up the attempt to picture a hill at all. A contour is the line along which the ground holds a single height, and a map with contours at a fixed interval turns a slope into a spacing: lines close together mean steep ground, lines far apart mean gentle ground, and the reader measures where before there was only an impression. [D]

The method was at work at sea long before it reached the land. Charts had joined soundings of equal depth since the seventeenth century, because a captain needs a number rather than a picture. Surveyors resisted it ashore for another century, partly because contouring meant levelling across a whole district instead of sketching from a viewpoint, and partly because the result looked, to trained eyes, more like a diagram than a map. What settled it was artillery. A gunner needed to know whether a ridge would hide him, and only a contoured sheet could answer that from a desk.