1r.slope.aspect(1) Grass User's Manual r.slope.aspect(1)
2
6 r.slope.aspect - Generates raster map layers of slope, aspect, curva‐
7 tures and partial derivatives from a raster map layer of true elevation
8 values. Aspect is calculated counterclockwise from east.
9
11 raster
12
14 r.slope.aspect
15 r.slope.aspect help
16 r.slope.aspect [-qa] elevation=string [slope=string] [aspect=string]
17 [format=string] [prec=string] [pcurv=string] [tcurv=string]
18 [dx=string] [dy=string] [dxx=string] [dyy=string] [dxy=string]
19 [zfactor=float] [min_slp_allowed=float] [--overwrite] [--verbose]
20 [--quiet]
21 Flags:
23 -q
24 Quiet
25 -a
27 Do not align the current region to the elevation layer
28 --overwrite
30 Allow output files to overwrite existing files
31 --verbose
33 Verbose module output
34 --quiet
36 Quiet module output
37 Parameters:
39 elevation=string
40 Raster elevation file name
41 slope=string
43 Output slope filename
44 aspect=string
46 Output aspect filename
47 format=string
49 Format for reporting the slope
50 Options: degrees,percent
51 Default: degrees
52 prec=string
54 Type of output aspect and slope maps
55 Options: default,double,float,int
56 Default: float
57 pcurv=string
59 Output profile curvature filename
60 tcurv=string
62 Output tangential curvature filename
63 dx=string
65 Output first order partial derivative dx (E-W slope) filename
66 dy=string
68 Output first order partial derivative dy (N-S slope) filename
69 dxx=string
71 Output second order partial derivative dxx filename
72 dyy=string
74 Output second order partial derivative dyy filename
75 dxy=string
77 Output second order partial derivative dxy filename
78 zfactor=float
80 Multiplicative factor to convert elevation units to meters
81 Default: 1.0
82 min_slp_allowed=float
84 Minimum slope val. (in percent) for which aspect is computed
85 Default: 0.0
86
88 r.slope.aspect generates raster maps of slope, aspect, curvatures and
89 first and second order partial derivatives from a raster map of true
90 elevation values. The user must specify the input elevation file name
91 and at least one output file name. The user can also specify the format
92 for slope (degrees, percent; default=degrees), and the zfactor: multi‐
93 plicative factor to convert elevation units to meters; (default 1.0).
94 The elevation input raster map specified by the user must contain true
96 elevation values, not rescaled or categorized data. If the elevation
97 values are in feet or other units than meters (with a conversion factor
98 meters:, defined in PROJ_UNITS), they must be converted to meters using
99 the parameter zfactor.
100 The aspect output raster map indicates the direction that slopes are
102 facing. The aspect categories represent the number degrees of east.
103 Category and color table files are also generated for the aspect map
104 layer. The aspect categories represent the number degrees of east and
105 they increase counterclockwise: 90deg is North, 180 is West, 270 is
106 South 360 is East. The aspect value 0 is used to indicate undefined
107 aspect in flat areas with slope=0.
108 The slope output raster map contains slope values, stated in degrees of
110 inclination from the horizontal if format=degrees option (the default)
111 is chosen, and in percent rise if format=percent option is chosen.
112 Category and color table files are generated.
113 Profile and tangential curvatures are the curvatures in the direction
115 of steepest slope and in the direction of the contour tangent respec‐
116 tively. The curvatures are expressed as 1/metres, e.g. a curvature of
117 0.05 corresponds to a radius of curvature of 20m.
118 For some applications, the user will wish to use a reclassified raster
120 map of slope that groups slope values into ranges of slope. This can be
121 done using r.reclass. An example of a useful reclassification is given
122 below:
123 (in degrees) (in percent)
124 1 0- 1 0- 2%
125 2 2- 3 3- 5%
126 3 4- 5 6- 10%
127 4 6- 8 11- 15%
128 5 9- 11 16- 20%
129 6 12- 14 21- 25%
130 7 15- 90 26% and higher
131 The following color table works well with the above
132 reclassification.
133 category red green blue
134 0 179 179 179
135 1 0 102 0
136 2 0 153 0
137 3 128 153 0
138 4 204 179 0
139 5 128 51 51
140 6 255 0 0
141 7 0 0 0
142
144 To ensure that the raster elevation map layer is not inappropriately
145 resampled, the settings for the current region are modified slightly
146 (for the execution of the program only): the resolution is set to match
147 the resolution of the elevation map and the edges of the region (i.e.
148 the north, south, east and west) are shifted, if necessary, to line up
149 along edges of the nearest cells in the elevation map. If the user
150 really wants the elevation map resampled to the current region resolu‐
151 tion, the -a flag should be specified.
152 The current mask is ignored.
154 The algorithm used to determine slope and aspect uses a 3x3 neighbor‐
156 hood around each cell in the elevation file. Thus, it is not possible
157 to determine slope and aspect for the cells adjacent to the edges in
158 the elevation map layer. These cells are assigned a "zero slope" value
159 (category 0) in both the slope and aspect raster map layers.
160 Horn's formula is used to find the derivatives in x and y directions.
162 Only when using integer elevation models, the aspect is biased in 0,
164 45, 90, 180, 225, 270, 315, and 360 directions; i.e., the distribution
165 of aspect categories is very uneven, with peaks at 0, 45,..., 360 cate‐
166 gories. When working with floating point elevation models, no such
167 aspect bias occurs.
168 Because most cells with a very small slope end up having category 0,
170 45, ..., 360, it is sometimes possible to reduce the bias in these
171 directions by filtering out the aspect in areas where the terrain is
172 almost flat. A new option min_slp_allowed was added to specify the min‐
173 imum slope for which aspect is computed. The aspect for all cells with
174 slope < min_slp_allowed is set to null.
175
177 Horn, B. K. P. (1981). Hill Shading and the Reflectance Map, Proceed‐
178 ings of the IEEE, 69(1):14-47.
179
181 r.mapcalc, r.neighbors, r.reclass, r.rescale
182
184 Michael Shapiro, U.S.Army Construction Engineering Research Laboratory
185 Olga Waupotitsch, U.S.Army Construction Engineering Research Laboratory
187 Last changed: $Date: 2006-11-19 09:50:25 +0100 (Sun, 19 Nov 2006) $
189 Full index
191 © 2003-2008 GRASS Development Team
193GRASS 6.3.0 r.slope.aspect(1)