Scratching Surfaces

There’s a known trick for creating an “antique looking” map as a background raster.  Starting from a DEM,  you create a slope raster, then give it a sepia color ramp. And, presto, a poor man’s antique effect.

Here’s how it’s done in GRASS, with a twist to improve the results slightly. Assuming the elevation raster is called ‘dem’:

GRASS 6.4.0RC5 (WGS84):~ > g.region -p rast=dem
GRASS 6.4.0RC5 (WGS84):~ > r.slope.aspect elevation=dem slope=slope
GRASS 6.4.0RC5 (WGS84):~ > r.colors -n slope color=sepia

Note the ‘-n’ switch to r.colors. This reverses the color ramp so that high slopes have a darker shade of sepia.  Have a look at the results so far. Now create a shaded relief map from the dem

GRASS 6.4.0RC5 (WGS84):~ > r.shaded.relief map=dem shade=dem_shade azimuth=315 zmult=1.5
GRASS 6.4.0RC5 (WGS84):~ > r.colors -n dem_shade color=sepia

And now the twist. Merge the slope raster with the shaded relief, to get an “antique 3d” effect. The GRASS r.blend module has a “percent” parameter which indicates how much of the first map is used in the mix.

GRASS 6.4.0RC5 (WGS84):~ > r.blend first=dem_shade second=slope percent=20 out=blend
GRASS 6.4.0RC5 (WGS84):~ > r.composite red=blend.r green=blend.g blue=blend.b out=slope_3d
GRASS 6.4.0RC5 (WGS84):~ > g.remove rast=`g.mlist rast pat=blend.* sep=,`

Display your slope_3d map and let me know what you think. (My example is based on DEM layers from the ASTER DEM project)

3D effect

Antique effect

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