Cartography Word of the Day: Hachure

An old and particularly “hairy” way of showing elevation and slope

A section of sheet 19 of the 1858 Dufour map of Switzerland, illustrating hachure shading for elevation detail (click on the image to see it larger).

One of the challenges of displaying landforms is analgous to the basic challenge of dislpaying a curved surface on a flat one with as little distortion as possible; showing physical 3-D relief on a flat surface requires some engineering and representation methods that do not occur in nature–the elevation lines (contours) certainly do not appear on the ground.

In 1799, an Austrian army officer, Johann Georg Lehmann, hit on an idea to combine graphical representation of slope and shading to generate an impression of depth: the hachure.

The hachure is simply a line. Its utility comes in how it’s applied. The essential hachure is a line of varying width, following the tendency of the slope in the direction of greatest slope; that is to say, a hachure crossing an contour would do so at a right angle. The width of this line would vary as the slope increased (get thicker) or decreased (get smaller). Hachure application did not necessarily use varying widths to indicate slope; some drafters would increase thier density to indicate greater slope. When varied uniformly, they can create the illusion of shading, thus giving a depth and reality to the landforms depcited.

While not seen widely today, hachured maps were widely used prior to the development of lithography, when it was possible to easily produce continuous tonal variation.

Reference: Elements of Cartography, 4th ed., Robinson, Sale, and Morrison. Wiley & Sons, New York, 1978