## 03 September 2013

### This is a "hachure map"

Somewhere this weekend I ran across a reference to a "hachure" on a map, and had to look it up.  I assumed it had something to do with "cross-hatching" for shading.  Not quite.
Hachures are an older mode of representing relief. They show orientation of slope, and by their thickness and overall density they provide a general sense of steepness. Being non-numeric, they are less useful to a scientific survey than contours, but can successfully communicate quite specific shapes of terrain. They are a form of shading, although different from the one used in shaded maps...

Hachures are strokes (short line segments or curves) drawn in the direction of the steepest slope (the aspect direction). Steeper slopes are represented by thicker, shorter strokes, while gentler slopes are represented by thinner, longer and farther apart strokes. A very gentle slope or a flat area, like the top of a hill, are usually left blank. The hachures are traditionally monocolour, usually black, gray or brown; using two complementary colours for the hachures on a neutral background colour (e.g. black and white lines on gray map colour) would give a shading effect as if the relief were illuminated.

In representing relief with hachures on a map, six rules are to be followed, according to G.R.P. Lawrence (1979):
The hachures are drawn in the direction of the steepest gradient.
The hachures are arranged in rows perpendicular to their direction.
The length and thickness of each stroke represents the drop in height along its direction: a short and thick stroke represents a short and steep slope, while a long and thin stroke represents a long and gentle slope.
The strokes are spaced at an equal distance inside a row.
The strokes have the same thickness inside a row.
If the map is illuminated, strokes are thinner and farther apart on the illuminated side.
You learn something every day.