1r.drain(1) GRASS GIS User's Manual r.drain(1)
2
6 r.drain - Traces a flow through an elevation model or cost surface on
7 a raster map.
8
10 raster, hydrology, cost surface
11
13 r.drain
14 r.drain --help
15 r.drain [-cand] input=name [direction=name] output=name [drain=name]
16 [start_coordinates=east,north[,east,north,...]]
17 [start_points=name[,name,...]] [--overwrite] [--help] [--verbose]
18 [--quiet] [--ui]
19 Flags:
21 -c
22 Copy input cell values on output
23 -a
25 Accumulate input values along the path
26 -n
28 Count cell numbers along the path
29 -d
31 The input raster map is a cost surface (direction surface must also
32 be specified)
33 --overwrite
35 Allow output files to overwrite existing files
36 --help
38 Print usage summary
39 --verbose
41 Verbose module output
42 --quiet
44 Quiet module output
45 --ui
47 Force launching GUI dialog
48 Parameters:
50 input=name [required]
51 Name of input elevation or cost surface raster map
52 direction=name
54 Name of input movement direction map associated with the cost sur‐
55 face
56 Direction in degrees CCW from east
57 output=name [required]
59 Name for output raster map
60 drain=name
62 Name for output drain vector map
63 Recommended for cost surface made using knight’s move
64 start_coordinates=east,north[,east,north,...]
66 Coordinates of starting point(s) (E,N)
67 start_points=name[,name,...]
69 Name of starting vector points map(s)
70
72 r.drain traces a flow through a least-cost path in an elevation model
73 or cost surface. For cost surfaces, a movement direction map must be
74 specified with the direction option and the -d flag to trace a flow
75 path following the given directions. Such a movement direction map can
76 be generated with r.walk, r.cost, r.slope.aspect or r.watershed pro‐
77 vided that the direction is in degrees, measured counterclockwise from
78 east.
79 The output raster map will show one or more least-cost paths between
81 each user-provided location(s) and the minima (low category values) in
82 the raster input map. If the -d flag is used the output least-cost
83 paths will be found using the direction raster map. By default, the
84 output will be an integer CELL map with category 1 along the least cost
85 path, and null cells elsewhere.
86 With the -c (copy) flag, the input raster map cell values are copied
88 verbatim along the path. With the -a (accumulate) flag, the accumulated
89 cell value from the starting point up to the current cell is written on
90 output. With either the -c or the -a flags, the output map is created
91 with the same cell type as the input raster map (integer, float or dou‐
92 ble). With the -n (number) flag, the cells are numbered consecutively
93 from the starting point to the final point. The -c, -a, and -n flags
94 are mutually incompatible.
95 For an elevation surface, the path is calculated by choosing the
97 steeper "slope" between adjacent cells. The slope calculation accu‐
98 rately accounts for the variable scale in lat-lon projections. For a
99 cost surface, the path is calculated by following the movement direc‐
100 tion surface back to the start point given in r.walk or r.cost. The
101 path search stops as soon as a region border or a neighboring NULL cell
102 is encountered, because in these cases the direction can not be deter‐
103 mined (the path could continue outside the current region).
104 The start_coordinates parameter consists of map E and N grid coordi‐
106 nates of a starting point. Each x,y pair is the easting and northing
107 (respectively) of a starting point from which a least-cost corridor
108 will be developed. The start_points parameter can take multiple vector
109 maps containing additional starting points. Up to 1024 starting points
110 can be input from a combination of the start_coordinates and
111 start_points parameters.
112 Explanation of output values
114 Consider the following example:
115 Input: Output:
116 ELEVATION SURFACE LEAST COST PATH
117 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
118 . 19. 20. 18. 19. 16. 15. 15. . . . . . . . .
119 . . --- . . . . . . . . . . . . . . . . . . . . . . . . .
120 . 20| 19| 17. 16. 17. 16. 16. . . 1 . 1 . 1 . . . .
121 . . --- . . . . . . . . . . . . . . . . . . . . . . . . .
122 . 18. 18. 24. 18. 15. 12. 11. . . . . . 1 . . .
123 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
124 . 22. 16. 16. 18. 10. 10. 10. . . . . . 1 . . .
125 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
126 . 17. 15. 15. 15. 10. 8 . 8 . . . . . . . 1 . .
127 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
128 . 24. 16. 8 . 7 . 8 . 0 . 12. . . . . . . 1 . .
129 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
130 . 17. 9 . 8 . 7 . 8 . 6 . 12. . . . . . . . .
131 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
132 The user-provided starting location in the above example is the boxed
134 19 in the left-hand map. The path in the output shows the least-cost
135 corridor for moving from the starting box to the lowest (smallest) pos‐
136 sible point. This is the path a raindrop would take in this landscape.
137 With the -c (copy) flag, you get the following result:
139 Input: Output:
140 ELEVATION SURFACE LEAST COST PATH
141 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
142 . 19. 20. 18. 19. 16. 15. 15. . . . . . . . .
143 . . --- . . . . . . . . . . . . . . . . . . . . . . . . .
144 . 20| 19| 17. 16. 17. 16. 16. . . 19. 17. 16. . . .
145 . . --- . . . . . . . . . . . . . . . . . . . . . . . . .
146 . 18. 18. 24. 18. 15. 12. 11. . . . . . 15. . .
147 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
148 . 22. 16. 16. 18. 10. 10. 10. . . . . . 10. . .
149 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
150 . 17. 15. 15. 15. 10. 8 . 8 . . . . . . . 8 . .
151 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
152 . 24. 16. 8 . 7 . 8 . 0 .12 . . . . . . . 0 . .
153 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
154 . 17. 9 . 8 . 7 . 8 . 6 .12 . . . . . . . . .
155 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
156 Note that the last 0 will not be put in the null values map.
157 With the -a (accumulate) flag, you get the following result:
159 Input: Output:
160 ELEVATION SURFACE LEAST COST PATH
161 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
162 . 19. 20. 18. 19. 16. 15. 15. . . . . . . . .
163 . . --- . . . . . . . . . . . . . . . . . . . . . . . . .
164 . 20| 19| 17. 16. 17. 16. 16. . . 19. 36. 52. . . .
165 . . --- . . . . . . . . . . . . . . . . . . . . . . . . .
166 . 18. 18. 24. 18. 15. 12. 11. . . . . . 67. . .
167 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
168 . 22. 16. 16. 18. 10. 10. 10. . . . . . 77. . .
169 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
170 . 17. 15. 15. 15. 10. 8 . 8 . . . . . . . 85. .
171 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
172 . 24. 16. 8 . 7 . 8 . 0 .12 . . . . . . . 85. .
173 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
174 . 17. 9 . 8 . 7 . 8 . 6 .12 . . . . . . . . .
175 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
176 With the -n (number) flag, you get the following result:
178 Input: Output:
179 ELEVATION SURFACE LEAST COST PATH
180 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
181 . 19. 20. 18. 19. 16. 15. 15. . . . . . . . .
182 . . --- . . . . . . . . . . . . . . . . . . . . . . . . .
183 . 20| 19| 17. 16. 17. 16. 16. . . 1 . 2 . 3 . . . .
184 . . --- . . . . . . . . . . . . . . . . . . . . . . . . .
185 . 18. 18. 24. 18. 15. 12. 11. . . . . . 4 . . .
186 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
187 . 22. 16. 16. 18. 10. 10. 10. . . . . . 5 . . .
188 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
189 . 17. 15. 15. 15. 10. 8 . 8 . . . . . . . 6 . .
190 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
191 . 24. 16. 8 . 7 . 8 . 0 .12 . . . . . . . 7 . .
192 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
193 . 17. 9 . 8 . 7 . 8 . 6 .12 . . . . . . . . .
194 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
195 With the -d (direction) flag, the direction raster is used for the
196 input, rather than the elevation surface. The output is then created
197 according to one of the -can flags.
198 The directions are recorded as degrees CCW from East:
199 112.5 67.5 i.e. a cell with the value 135
200 157.5 135 90 45 22.5 means the next cell is to the North-West
201 180 x 0
202 202.5 225 270 315 337.5
203 247.5 292.5
204
206 If no direction input map is given, r.drain currently finds only the
207 lowest point (the cell having the smallest category value) in the input
208 file that can be reached through directly adjacent cells that are less
209 than or equal in value to the cell reached immediately prior to it;
210 therefore, it will not necessarily reach the lowest point in the input
211 file. It currently finds pits in the data, rather than the lowest point
212 in the entire input map. The r.fill.dir, r.terraflow, and r.basins.fill
213 modules can be used to fill in subbasins prior to processing with
214 r.drain.
215 r.drain will not give sane results at the region boundary. On outer
217 rows and columns bordering the edge of the region, the flow direction
218 is always directly out of the map. In this case, the user could try
219 adjusting the region extents slightly with g.region to allow additional
220 outlet paths for r.drain.
221
223 Path to the lowest point
224 In this example we compute drainage paths from two given points follow‐
225 ing decreasing elevation values to the lowest point. We are using the
226 full North Carolina sample dataset. First we create the two points
227 from a text file using v.in.ascii module (here the text file is CSV and
228 we are using unix here-file syntax with EOF, in GUI just enter the val‐
229 ues directly for the parameter input):
230 v.in.ascii input=- output=start format=point separator=comma <<EOF
231 638667.15686275,220610.29411765
232 638610.78431373,220223.03921569
233 EOF
234 Now we compute the drainage path:
235 r.drain input=elev_lid792_1m output=drain_path drain=drain start_points=start
236 Before we visualize the result, we set a color table for the elevation
237 we are using and we create a shaded relief map:
238 r.colors map=elev_lid792_1m color=elevation
239 r.relief input=elev_lid792_1m output=relief
240 Finally we visualize all the input and output data:
241 d.shade shade=relief color=elev_lid792_1m
242 d.vect map=drain_path color=0:0:61 width=4 legend_label="drainage paths"
243 d.vect map=start color=none fill_color=224:0:0 icon=basic/circle size=15 legend_label=origins
244 d.legend.vect -b
245 Figure: Drainage paths from two points flowing into the points with
246 lowest values
247 Path following directions
249 To continue flow even after it hits a depression, we need to supply a
250 direction raster map which will tell the r.drain module how to continue
251 from the depression. To get these directions, we use the r.watershed
252 module:
253 r.watershed elevation=elev_lid792_1m accumulation=accum drainage=drain_dir
254 The directions are categorical and we convert them to degrees using
255 raster algebra:
256 r.mapcalc "drain_deg = if(drain_dir != 0, 45. * abs(drain_dir), null())"
257 Together with directions, we need to provide the r.drain module with
258 cost values. We don’t have any cost to assign to specific cells, so we
259 create a constant surface:
260 r.mapcalc "const1 = 1"
261 Now we are ready to compute the drainage paths. We are using the two
262 points from the previous example.
263 r.drain -d input=const1 direction=drain_deg output=drain_path_2 drain=drain_2 start_points=start
264 We visualize the result in the same way as in the previous example.
265 Figure: Drainage paths from two points where directions from r.water‐
266 shed were used
267
269 Sometimes, when the differences among integer cell category values in
270 the r.cost cumulative cost surface output are small, this cumulative
271 cost output cannot accurately be used as input to r.drain (r.drain will
272 output bad results). This problem can be circumvented by making the
273 differences between cell category values in the cumulative cost output
274 bigger. It is recommended that if the output from r.cost is to be used
275 as input to r.drain, the user multiply the r.cost input cost surface
276 map by the value of the map’s cell resolution, before running r.cost.
277 This can be done using r.mapcalc. The map resolution can be found using
278 g.region. This problem doesn’t arise with floating point maps.
279
281 g.region, r.cost, r.fill.dir, r.basins.fill, r.watershed, r.terraflow,
282 r.mapcalc, r.walk
283
285 Completely rewritten by Roger S. Miller, 2001
286 July 2004 at WebValley 2004, error checking and vector points added by
287 Matteo Franchi (Liceo Leonardo Da Vinci, Trento) and Roberto Flor
288 (ITC-irst, Trento, Italy)
289
291 Available at: r.drain source code (history)
292 Main index | Raster index | Topics index | Keywords index | Graphical
294 index | Full index
295 © 2003-2020 GRASS Development Team, GRASS GIS 7.8.5 Reference Manual
297GRASS 7.8.5 r.drain(1)