1r.slope.aspect(1) Grass User's Manual r.slope.aspect(1)
2
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.
9
11 raster, terrain, aspect, slope, curvature
12
14 r.slope.aspect
15 r.slope.aspect --help
16 r.slope.aspect [-a] 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 Flags:
23 -a
24 Do not align the current region to the raster elevation map
25 --overwrite
27 Allow output files to overwrite existing files
28 --help
30 Print usage summary
31 --verbose
33 Verbose module output
34 --quiet
36 Quiet module output
37 --ui
39 Force launching GUI dialog
40 Parameters:
42 elevation=name [required]
43 Name of input elevation raster map
44 slope=name
46 Name for output slope raster map
47 aspect=name
49 Name for output aspect raster map
50 format=string
52 Format for reporting the slope
53 Options: degrees, percent
54 Default: degrees
55 precision=string
57 Type of output aspect and slope maps
58 Storage type for resultant raster map
59 Options: CELL, FCELL, DCELL
60 Default: FCELL
61 CELL: Integer
62 FCELL: Single precision floating point
63 DCELL: Double precision floating point
64 pcurvature=name
66 Name for output profile curvature raster map
67 tcurvature=name
69 Name for output tangential curvature raster map
70 dx=name
72 Name for output first order partial derivative dx (E-W slope)
73 raster map
74 dy=name
76 Name for output first order partial derivative dy (N-S slope)
77 raster map
78 dxx=name
80 Name for output second order partial derivative dxx raster map
81 dyy=name
83 Name for output second order partial derivative dyy raster map
84 dxy=name
86 Name for output second order partial derivative dxy raster map
87 zscale=float
89 Multiplicative factor to convert elevation units to horizontal
90 units
91 Default: 1.0
92 min_slope=float
94 Minimum slope value (in percent) for which aspect is computed
95 Default: 0.0
96
98 r.slope.aspect generates raster maps of slope, aspect, curvatures and
99 first and second order partial derivatives from a raster map of true
100 elevation values. The user must specify the input elevation raster map
101 and at least one output raster maps. The user can also specify the for‐
102 mat for slope (degrees, percent; default=degrees), and the zscale: mul‐
103 tiplicative factor to convert elevation units to horizontal units;
104 (default 1.0).
105 The elevation input raster map specified by the user must contain true
107 elevation values, not rescaled or categorized data. If the elevation
108 values are in other units than in the horizontal units, they must be
109 converted to horizontal units using the parameter zscale. In GRASS GIS
110 7, vertical units are not assumed to be meters any more. For example,
111 if both your vertical and horizontal units are feet, parameter zscale
112 must not be used.
113 The aspect output raster map indicates the direction that slopes are
115 facing. The aspect categories represent the number degrees of east.
116 Category and color table files are also generated for the aspect raster
117 map. The aspect categories represent the number degrees of east and
118 they increase counterclockwise: 90 degrees is North, 180 is West, 270
119 is South 360 is East.
120 Note: These values can be transformed to azimuth (0 is North, 90 is
121 East, etc) values using r.mapcalc:
122 # convert angles from CCW to north up
123 r.mapcalc "azimuth_aspect = (450 - ccw_aspect) % 360"
124 The aspect is not defined for slope equal to zero. Thus, most cells
126 with a very small slope end up having category 0, 45, ..., 360 in
127 aspect output. It is possible to reduce the bias in these directions
128 by filtering out the aspect in areas where the terrain is almost flat.
129 A option min_slope can be used to specify the minimum slope for which
130 aspect is computed. The aspect for all cells with slope < min_slope is
131 set to null (no-data).
132 The slope output raster map contains slope values, stated in degrees of
134 inclination from the horizontal if format=degrees option (the default)
135 is chosen, and in percent rise if format=percent option is chosen.
136 Category and color table files are generated.
137 Profile and tangential curvatures are the curvatures in the direction
139 of steepest slope and in the direction of the contour tangent respec‐
140 tively. The curvatures are expressed as 1/metres, e.g. a curvature of
141 0.05 corresponds to a radius of curvature of 20m. Convex form values
142 are positive and concave form values are negative.
143 Example DEM
145 Slope (degree) from example DEM Aspect (degree) from example DEM
147 Tangential curvature (m-1) from example DEM Profile curvature (m-1) from example DEM
149 For some applications, the user will wish to use a reclassified raster
152 map of slope that groups slope values into ranges of slope. This can be
153 done using r.reclass. An example of a useful reclassification is given
154 below:
155 category range category labels
156 (in degrees) (in percent)
157 1 0- 1 0- 2%
158 2 2- 3 3- 5%
159 3 4- 5 6- 10%
160 4 6- 8 11- 15%
161 5 9- 11 16- 20%
162 6 12- 14 21- 25%
163 7 15- 90 26% and higher
164 The following color table works well with the above
165 reclassification.
166 category red green blue
167 0 179 179 179
168 1 0 102 0
169 2 0 153 0
170 3 128 153 0
171 4 204 179 0
172 5 128 51 51
173 6 255 0 0
174 7 0 0 0
175
177 To ensure that the raster elevation map is not inappropriately resam‐
178 pled, the settings for the current region are modified slightly (for
179 the execution of the program only): the resolution is set to match the
180 resolution of the elevation raster map and the edges of the region
181 (i.e. the north, south, east and west) are shifted, if necessary, to
182 line up along edges of the nearest cells in the elevation map. If the
183 user really wants the raster elevation map resampled to the current
184 region resolution, the -a flag should be specified.
185 The current mask is ignored.
187 The algorithm used to determine slope and aspect uses a 3x3 neighbor‐
189 hood around each cell in the raster elevation map. Thus, it is not pos‐
190 sible to determine slope and aspect for the cells adjacent to the edges
191 in the elevation map layer. These cells are assigned a "zero slope"
192 value (category 0) in both the slope and aspect raster maps.
193 Horn’s formula is used to find the first order derivatives in x and y
195 directions.
196 Only when using integer elevation models, the aspect is biased in 0,
198 45, 90, 180, 225, 270, 315, and 360 directions; i.e., the distribution
199 of aspect categories is very uneven, with peaks at 0, 45,..., 360 cate‐
200 gories. When working with floating point elevation models, no such
201 aspect bias occurs.
202
204 Calculation of slope, aspect, profile and tangential curvature
205 In this example a slope, aspect, profile and tangential curvature map
206 are computed from an elevation raster map (North Carolina sample
207 dataset):
208 g.region raster=elevation
209 r.slope.aspect elevation=elevation slope=slope aspect=aspect pcurvature=pcurv tcurvature=tcurv
210 # set nice color tables for output raster maps
211 r.colors -n map=slope color=sepia
212 r.colors map=aspect color=aspectcolr
213 r.colors map=pcurv color=curvature
214 r.colors map=tcurv color=curvature
215 Figure: Slope, aspect, profile and tangential curvature raster map
217 (North Carolina dataset)
218 Classification of major aspect directions in compass orientation
220 In the following example (based on the North Carolina sample dataset)
221 we first generate the standard aspect map (oriented CCW from East),
222 then convert it to compass orientation, and finally classify four major
223 aspect directions (N, E, S, W):
224 g.region raster=elevation -p
225 # generate aspect map with CCW orientation
226 r.slope.aspect elevation=elevation aspect=myaspect
227 # generate compass orientation and classify four major directions (N, E, S, W)
228 r.mapcalc "aspect_4_directions = eval( \\
229 compass=(450 - myaspect ) % 360, \\
230 if(compass >=0. && compass < 45., 1) \\
231 + if(compass >=45. && compass < 135., 2) \\
232 + if(compass >=135. && compass < 225., 3) \\
233 + if(compass >=225. && compass < 315., 4) \\
234 + if(compass >=315., 1) \\
235 )"
236 # assign text labels
237 r.category aspect_4_directions separator=comma rules=- << EOF
238 1,north
239 2,east
240 3,south
241 4,west
242 EOF
243 # assign color table
244 r.colors aspect_4_directions rules=- << EOF
245 1 253,184,99
246 2 178,171,210
247 3 230,97,1
248 4 94,60,153
249 EOF
250 Aspect map classified to four major compass directions (zoomed subset
251 shown)
252
254 · Horn, B. K. P. (1981). Hill Shading and the Reflectance Map,
255 Proceedings of the IEEE, 69(1):14-47.
256 · Mitasova, H. (1985). Cartographic aspects of computer surface
258 modeling. PhD thesis. Slovak Technical University , Bratislava
259 · Hofierka, J., Mitasova, H., Neteler, M., 2009. Geomorphometry
261 in GRASS GIS. In: Hengl, T. and Reuter, H.I. (Eds), Geomor‐
262 phometry: Concepts, Software, Applications. Developments in
263 Soil Science, vol. 33, Elsevier, 387-410 pp, http://www.geomor‐
264 phometry.org
265
267 r.mapcalc, r.neighbors, r.reclass, r.rescale
268
270 Michael Shapiro, U.S.Army Construction Engineering Research Laboratory
271 Olga Waupotitsch, U.S.Army Construction Engineering Research Laboratory
272 Last changed: $Date: 2015-08-12 11:45:29 +0200 (Wed, 12 Aug 2015) $
274
276 Available at: r.slope.aspect source code (history)
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280 © 2003-2019 GRASS Development Team, GRASS GIS 7.4.4 Reference Manual
282GRASS 7.4.4 r.slope.aspect(1)