Hachure map

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. Hachure representation of relief was standardized by the Austrian topographer Johann Georg Lehmann in 1799 [ [http://www.esri.com/news/arcuser/0701/althillshade.html Hillshading Alternatives with ArcGIS] ] . Hachures may be combined with other ways of representing relief, like shades, the result being a shaded hachure map; an example of such maps are the Dufour maps of Switzerland [ [http://www.swisstopo.admin.ch/internet/swisstopo/en/home/services/cartography/collection/dufour.html Dufour Map from the The Federal Office of Topography of Switzerland] ] . Emil von Sydow designed maps with coloured hachures: green for lowlands and brown for highlands.

Overview

Hachures are strokes (short line segments or curves) drawn in the direction of the steepest slope (the aspect direction) [ [http://www.esri.com/news/arcuser/0701/althillshade.html Hillshading Alternatives with ArcGIS] ] . Steeper slopes are represented by thicker, shorter and closer 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, grey or brown; using two complementary colours for the hachures on a neutral background colour (e.g. black and white lines on grey map colour) would give a shading effect as if the relief were illuminated.

Rules

In representing relief with hachures on a map, six rules are to be followed, according to G.R.P. Lawrence (1979) [ [http://books.google.ro/books?id=YWaOBSjevD0C&pg=PA548&lpg=PA548&dq=%22hachure+map%22&source=web&ots=VM-_12wVlW&sig=0VGyCje1L-bt5i2CNHpeAroRadU&hl=ro#PPA548,M1 "The Guild Handbook of Scientific Illustration"] , 2nd edition, edited by Elaine R. S. Hodges, John Wiley and Sons, 2003 (rules are from G.R.P. Lawrence, "Cartographic Methods", 2nd edition, Methuen, 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.

The Swiss cartographer Eduard Imhof set 5 similar rules [http://www.mbmg.mtech.edu/pdf/gis_hachuretxt.pdf Patrick J. Kennelly, A. Jon Kimerling, Desktop Hachure Maps from Digital Elevation Models (cites E. Imhof, Cartographic Relief Presentation, Berlin and New York: Walter de Gruyter, 1982)] :

# Hachures follow the direction of steepest gradient
# Hachures are arranged in horizontal rows
# Hachure length corresponds to the local horizontal distance between assumed contours of a certain interval
# Hachure width is thicker for steeper slopes
# Hachure density remains constant throughout the map area.

If the illumination is vertical, rule 5 is kept; in the case of oblique illumination, it is dropped. The rules above are to be obeyed for large-scale maps. If the map being drawn is a small-scale map (less than 1:500 000 according to Imhof), rules may be relaxed in order to obtain a more suggestive representation.

References

External links

* [http://www.mbmg.mtech.edu/pdf/gis_hachuretxt.pdf Desktop Hachure Maps from Digital Elevation Models]
* [http://www.geog.port.ac.uk/webmap/hantscat/html/mapmenu.htm Archive of Hampshire Maps at the Geography Department of the University of Portsmouth]
* [http://www.geog.port.ac.uk/webmap/hantsmap/hantsmap/ordnce6/oss42.htm Map of part of Hampshire, 1810s]
* [http://www.geog.port.ac.uk/webmap/hantsmap/hantsmap/grnwood2/grw64.htm Map of Hampshire, 1826]