Raster Meets Point Data - Open Source Geospatial Tools: Applications in Earth Observation - page 155

(a) (b)

Height (m)
1000
1375
1750
2125
2500

0 500 1000 m 0 500 1000 m

Fig. 10.6 Slope combined with relief mode applied to Mount St. Helens using gdaldem . a Digital
surface model. b Slope relief

degrees (e.g. Lat/Long WGS84 projection), you can use scale = 111120 if the
vertical units are meters (or scale = 370400 if they are in feet).

For the next example, we first calculate the slope of the DSMof Mount St. Helens.
The resulting is a floating point image where each pixel value represents the steep-
ness of the terrain in degrees. We then use gdaldem in color-relief mode
to generate a color relief map, using various shades of gray (shown in Fig. 10.6 ).
The shades are automatically interpolated between the extreme values for 0 ⓦ (flat
terrain is white) and 90 ⓦ (steep terrain is black). For these two extreme values, we
provide the following color table as a text file (save these lines in slope_ct.txt ).

0 255 255 255
90000

gdaldem slope st-helens_dsm_10m.tif st-helens_slope.tif
gdaldem color-relief st-helens_slope.tif slope_ct.txt
ₒ

st-helens_slope_relief.tif

10.6.3 Aspect

Aspect is an analysis mode that calculates the orientation (azimuth) of the slopes in
the DEM. Slopes can face North (0 ⓦ ), East (90 ⓦ ), South (180 ⓦ ), West (270 ⓦ )orany
other direction in between. Instead of azimuth angles, a trigonometric angle can be