22) Two developments were crucial: indirectly, the invention of the contour line (curved lines of equivalent elevation first used to show the overall form of landmasses on a French map of (1791) and, directly, Johann Georg Lehmann's 1799 invention of shadow-hachures, an arrangement of hachures that emphasized the "transition from level ground to steep gradients.
A tangle of squiggles denoting either a river or a slope face of indeterminate orientation touches the arc of the road symbol before it disappears into the dark interlineations of letters and hachures on the way to Italy.
On a planimetric map, hachures are linear symbols drawn perpendicular to contours and represent paths of steepest slope across a surface patch.
The first technique, rendering hachures as lines of steepest slope, adds reasonable slope information to a landscape illustration rendering hut does not imitate typical manual practice.
The second strategy, represented by algorithm Build Slope Lines, extends Build Hachures with a recursive technique that still renders hachures as straight lines but limits their rendering to triangles falling within a specified topological distance from a crease.