Raster Meets Point Data - Open Source Geospatial Tools: Applications in Earth Observation - page 154

(a) (b)

Height (m)
1000
1375
1750
2125
2500

0 500 1000 m 0 500 1000 m

Fig. 10.5 Hillshade applied to Mount St. Helens using gdaldem . a Digital surface model.
b Hillshade visualization mode

-alt altitude
altitude of the light, in degrees. 90 if the light comes from above the DEM, 0 if it
is raking light.
-combined combined shading
(from GDAL 1.10) a combination of slope and oblique shading.

An example of the hillshade visualization mode is shown in Fig. 10.5 . The altitude
of the light source is set to 50 ⓦ .

gdaldem hillshade -alt 50 st-helens_dsm_10m.tif
ₒ st-helens_hillshade.tif

10.6.2 Slope

Slope is an analysismode tomeasure the slope (steepness) of the terrain by comparing
the elevation of each pixel to its neighboring pixels (in a 3 3 window). The output

is in degrees or percentage (option -p ) and stored as a Float32. You can scale the
vertical units (elevation). The no-data value is fixed to

×

− 9999.

-p

If specified, the slope will be expressed as percent slope. Otherwise, it is expressed
as degrees
-s scale
ratio of vertical units to horizontal. If the horizontal unit of the source DEM is