Raster Meets Point Data - Open Source Geospatial Tools: Applications in Earth Observation - page 156

90 ⓦ . Flat areas
have no orientation (but vertically) and are assigned a no-data value, which is fixed
to

= −

selected (option -trigonometric ): trigonometric azimuth

− 9999. The output data type is a Float32.

-trigonometric
Return trigonometric angle instead of azimuth. Thus 0 ⓦ means East, 90 ⓦ North,
180 ⓦ West and 270 ⓦ South.
-zero_for_flat
return 0 for flat areas with slope = 0, instead of − 9999.

In Fig. 10.7 , we show the orientation of the slopes of Mount St. Helens as a result
of the following command:

gdaldem aspect st-helens_dsm_10m.tif st-helens_aspect.tif

10.6.4 Color-Relief

Color-relief is a visualization mode that produces a RGB (or RGBA) raster Byte
image, based on the DEM input and color table in text format. This text file contains
four columns: one for elevation and three for the RGB codes. Optionally, a fifth
column is used for transparency (RGBA). Other color formats are also supported,
including the GMT .cpt color palette tables (from GDAL 1.8.0), r.colors (GRASS)
and .clr (ESRI). A selected number of named colors can also be used. Unlike the
indexed color images discussed in Chap. 6 , the color table must not be complete (one

(a) (b)

Height (m)
1000
1375
1750
2125
2500

aspect
0
120
240
360

0 500 1000 m 0 500 1000 m

Fig. 10.7 Aspect applied to Mount St. Helens using gdaldem . a Digital surface model. b Aspect
analysis mode