1r.slope.aspect(1) GRASS GIS User's Manual r.slope.aspect(1)
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6 r.slope.aspect - Generates raster maps of slope, aspect, curvatures
7 and partial derivatives from an elevation raster map.
8 Aspect is calculated counterclockwise from east.
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11 raster, terrain, aspect, slope, curvature
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14 r.slope.aspect
15 r.slope.aspect --help
16 r.slope.aspect [-aen] elevation=name [slope=name] [aspect=name]
17 [format=string] [precision=string] [pcurvature=name] [tcurva‐
18 ture=name] [dx=name] [dy=name] [dxx=name] [dyy=name]
19 [dxy=name] [zscale=float] [min_slope=float] [--overwrite]
20 [--help] [--verbose] [--quiet] [--ui]
21
22 Flags:
23 -a
24 Do not align the current region to the raster elevation map
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26 -e
27 Compute output at edges and near NULL values
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29 -n
30 Create aspect as degrees clockwise from North (azimuth), with flat
31 = -9999
32 Default: degrees counter-clockwise from East, with flat = 0
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34 --overwrite
35 Allow output files to overwrite existing files
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37 --help
38 Print usage summary
39
40 --verbose
41 Verbose module output
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43 --quiet
44 Quiet module output
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46 --ui
47 Force launching GUI dialog
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49 Parameters:
50 elevation=name [required]
51 Name of input elevation raster map
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53 slope=name
54 Name for output slope raster map
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56 aspect=name
57 Name for output aspect raster map
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59 format=string
60 Format for reporting the slope
61 Options: degrees, percent
62 Default: degrees
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64 precision=string
65 Type of output aspect and slope maps
66 Storage type for resultant raster map
67 Options: CELL, FCELL, DCELL
68 Default: FCELL
69 CELL: Integer
70 FCELL: Single precision floating point
71 DCELL: Double precision floating point
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73 pcurvature=name
74 Name for output profile curvature raster map
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76 tcurvature=name
77 Name for output tangential curvature raster map
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79 dx=name
80 Name for output first order partial derivative dx (E-W slope)
81 raster map
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83 dy=name
84 Name for output first order partial derivative dy (N-S slope)
85 raster map
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87 dxx=name
88 Name for output second order partial derivative dxx raster map
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90 dyy=name
91 Name for output second order partial derivative dyy raster map
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93 dxy=name
94 Name for output second order partial derivative dxy raster map
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96 zscale=float
97 Multiplicative factor to convert elevation units to horizontal
98 units
99 Default: 1.0
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101 min_slope=float
102 Minimum slope value (in percent) for which aspect is computed
103 Default: 0.0
104
106 r.slope.aspect generates raster maps of slope, aspect, curvatures and
107 first and second order partial derivatives from a raster map of true
108 elevation values. The user must specify the input elevation raster map
109 and at least one output raster maps. The user can also specify the for‐
110 mat for slope (degrees, percent; default=degrees), and the zscale: mul‐
111 tiplicative factor to convert elevation units to horizontal units;
112 (default 1.0).
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114 The elevation input raster map specified by the user must contain true
115 elevation values, not rescaled or categorized data. If the elevation
116 values are in other units than in the horizontal units, they must be
117 converted to horizontal units using the parameter zscale. In GRASS GIS
118 7, vertical units are not assumed to be meters any more. For example,
119 if both your vertical and horizontal units are feet, parameter zscale
120 must not be used.
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122 The aspect output raster map indicates the direction that slopes are
123 facing counterclockwise from East: 90 degrees is North, 180 is West,
124 270 is South, 360 is East. Zero aspect indicates flat areas with zero
125 slope. Category and color table files are also generated for the aspect
126 raster map.
127 Note: These values can be transformed to azimuth values (90 is East,
128 180 is South, 270 is West, 360 is North) using r.mapcalc:
129 # convert angles from CCW from East to CW from North
130 # modulus (%) can not be used with floating point aspect values
131 r.mapcalc "azimuth_aspect = if(ccw_aspect == 0, 0, \
132 if(ccw_aspect < 90, 90 - ccw_aspect, \
133 450 - ccw_aspect)))"
134 Alternatively, the -n flag can be used to produce aspect as degrees CW
135 from North. Aspect for flat areas is then set to -9999 (default: 0).
136 Note: The reason for using -9999 is to be compliant with gdaldem which
137 uses -9999 by default as the nodata value.
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139 The aspect for slope equal to zero (flat areas) is set to zero (-9999
140 with -n flag). Thus, most cells with a very small slope end up having
141 category 0, 45, ..., 360 in aspect output. It is possible to reduce the
142 bias in these directions by filtering out the aspect in areas where the
143 terrain is almost flat. A option min_slope can be used to specify the
144 minimum slope for which aspect is computed. For all cells with
145 slope < min_slope, both slope and aspect are set to zero.
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147 The slope output raster map contains slope values, stated in degrees of
148 inclination from the horizontal if format=degrees option (the default)
149 is chosen, and in percent rise if format=percent option is chosen.
150 Category and color table files are generated.
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152 Profile and tangential curvatures are the curvatures in the direction
153 of steepest slope and in the direction of the contour tangent respec‐
154 tively. The curvatures are expressed as 1/metres, e.g. a curvature of
155 0.05 corresponds to a radius of curvature of 20m. Convex form values
156 are positive and concave form values are negative.
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158 Example DEM
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160 Slope (degree) from example DEM Aspect (degree) from example DEM
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162 Tangential curvature (m-1) from example DEM Profile curvature (m-1) from example DEM
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165 For some applications, the user will wish to use a reclassified raster
166 map of slope that groups slope values into ranges of slope. This can be
167 done using r.reclass. An example of a useful reclassification is given
168 below:
169 category range category labels
170 (in degrees) (in percent)
171 1 0- 1 0- 2%
172 2 2- 3 3- 5%
173 3 4- 5 6- 10%
174 4 6- 8 11- 15%
175 5 9- 11 16- 20%
176 6 12- 14 21- 25%
177 7 15- 90 26% and higher
178 The following color table works well with the above
179 reclassification.
180 category red green blue
181 0 179 179 179
182 1 0 102 0
183 2 0 153 0
184 3 128 153 0
185 4 204 179 0
186 5 128 51 51
187 6 255 0 0
188 7 0 0 0
189
191 To ensure that the raster elevation map is not inappropriately resam‐
192 pled, the settings for the current region are modified slightly (for
193 the execution of the program only): the resolution is set to match the
194 resolution of the elevation raster map and the edges of the region
195 (i.e. the north, south, east and west) are shifted, if necessary, to
196 line up along edges of the nearest cells in the elevation map. If the
197 user really wants the raster elevation map resampled to the current
198 region resolution, the -a flag should be specified.
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200 The current mask is ignored.
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202 The algorithm used to determine slope and aspect uses a 3x3 neighbor‐
203 hood around each cell in the raster elevation map. Thus, slope and
204 aspect are not determineed for cells adjacent to the edges and NULL
205 cells in the elevation map layer. These cells are by default set to
206 nodata in output raster maps. With the -e flag, output values are esti‐
207 mated for these cells, avoiding cropping along the edges.
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209 Horn’s formula is used to find the first order derivatives in x and y
210 directions.
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212 Only when using integer elevation models, the aspect is biased in 0,
213 45, 90, 180, 225, 270, 315, and 360 directions; i.e., the distribution
214 of aspect categories is very uneven, with peaks at 0, 45,..., 360 cate‐
215 gories. When working with floating point elevation models, no such
216 aspect bias occurs.
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219 Calculation of slope, aspect, profile and tangential curvature
220 In this example a slope, aspect, profile and tangential curvature map
221 are computed from an elevation raster map (North Carolina sample
222 dataset):
223 g.region raster=elevation
224 r.slope.aspect elevation=elevation slope=slope aspect=aspect pcurvature=pcurv tcurvature=tcurv
225 # set nice color tables for output raster maps
226 r.colors -n map=slope color=sepia
227 r.colors map=aspect color=aspectcolr
228 r.colors map=pcurv color=curvature
229 r.colors map=tcurv color=curvature
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231 Figure: Slope, aspect, profile and tangential curvature raster map
232 (North Carolina dataset)
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234 Classification of major aspect directions in compass orientation
235 In the following example (based on the North Carolina sample dataset)
236 we first generate the standard aspect map (oriented CCW from East),
237 then convert it to compass orientation, and finally classify four major
238 aspect directions (N, E, S, W):
239 g.region raster=elevation -p
240 # generate integer aspect map with degrees CCW from East
241 r.slope.aspect elevation=elevation aspect=myaspect precision=CELL
242 # generate compass orientation and classify four major directions (N, E, S, W)
243 r.mapcalc "aspect_4_directions = eval( \\
244 compass=(450 - myaspect ) % 360, \\
245 if(compass >=0. && compass < 45., 1) \\
246 + if(compass >=45. && compass < 135., 2) \\
247 + if(compass >=135. && compass < 225., 3) \\
248 + if(compass >=225. && compass < 315., 4) \\
249 + if(compass >=315., 1) \\
250 )"
251 # assign text labels
252 r.category aspect_4_directions separator=comma rules=- << EOF
253 1,north
254 2,east
255 3,south
256 4,west
257 EOF
258 # assign color table
259 r.colors aspect_4_directions rules=- << EOF
260 1 253,184,99
261 2 178,171,210
262 3 230,97,1
263 4 94,60,153
264 EOF
265 Aspect map classified to four major compass directions (zoomed subset
266 shown)
267
269 · Horn, B. K. P. (1981). Hill Shading and the Reflectance Map,
270 Proceedings of the IEEE, 69(1):14-47.
271
272 · Mitasova, H. (1985). Cartographic aspects of computer surface
273 modeling. PhD thesis. Slovak Technical University , Bratislava
274
275 · Hofierka, J., Mitasova, H., Neteler, M., 2009. Geomorphometry
276 in GRASS GIS. In: Hengl, T. and Reuter, H.I. (Eds), Geomor‐
277 phometry: Concepts, Software, Applications. Developments in
278 Soil Science, vol. 33, Elsevier, 387-410 pp, http://www.geomor‐
279 phometry.org
280
282 r.mapcalc, r.neighbors, r.reclass, r.rescale
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285 Michael Shapiro, U.S.Army Construction Engineering Research Laboratory
286 Olga Waupotitsch, U.S.Army Construction Engineering Research Laboratory
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289 Available at: r.slope.aspect source code (history)
290
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293
294 © 2003-2020 GRASS Development Team, GRASS GIS 7.8.5 Reference Manual
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298GRASS 7.8.5 r.slope.aspect(1)