1r.terraflow(1) Grass User's Manual r.terraflow(1)
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6 r.terraflow - Performs flow computation for massive grids.
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9 raster, hydrology, flow, accumulation, sink
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12 r.terraflow
13 r.terraflow --help
14 r.terraflow [-s] elevation=name [filled=name] [direction=name]
15 [swatershed=name] [accumulation=name] [tci=name] [d8cut=float]
16 [memory=integer] [directory=string] [stats=string] [--overwrite]
17 [--help] [--verbose] [--quiet] [--ui]
18 Flags:
20 -s
21 SFD (D8) flow (default is MFD)
22 SFD: single flow direction, MFD: multiple flow direction
23 --overwrite
25 Allow output files to overwrite existing files
26 --help
28 Print usage summary
29 --verbose
31 Verbose module output
32 --quiet
34 Quiet module output
35 --ui
37 Force launching GUI dialog
38 Parameters:
40 elevation=name [required]
41 Name of input elevation raster map
42 filled=name
44 Name for output filled (flooded) elevation raster map
45 direction=name
47 Name for output flow direction raster map
48 swatershed=name
50 Name for output sink-watershed raster map
51 accumulation=name
53 Name for output flow accumulation raster map
54 tci=name
56 Name for output topographic convergence index (tci) raster map
57 d8cut=float
59 Routing using SFD (D8) direction
60 If flow accumulation is larger than this value it is routed using
61 SFD (D8) direction (meaningful only for MFD flow). If no answer is
62 given it defaults to infinity.
63 memory=integer
65 Maximum memory to be used (in MB)
66 Default: 300
67 directory=string
69 Directory to hold temporary files (they can be large)
70 stats=string
72 Name for output file containing runtime statistics
73
75 r.terraflow takes as input a raster digital elevation model (DEM) and
76 computes the flow direction raster and the flow accumulation raster, as
77 well as the flooded elevation raster, sink-watershed raster (partition
78 into watersheds around sinks) and TCI (topographic convergence index)
79 raster maps.
80 r.terraflow computes these rasters using well-known approaches, with
82 the difference that its emphasis is on the computational complexity of
83 the algorithms, rather than on modeling realistic flow. r.terraflow
84 emerged from the necessity of having scalable software able to process
85 efficiently very large terrains. It is based on theoretically optimal
86 algorithms developed in the framework of I/O-efficient algorithms.
87 r.terraflow was designed and optimized especially for massive grids and
88 is able to process terrains which were impractical with similar func‐
89 tions existing in other GIS systems.
90 Flow directions are computed using either the MFD (Multiple Flow Direc‐
92 tion) model or the SFD (Single Flow Direction, or D8) model, illus‐
93 trated below. Both methods compute downslope flow directions by
94 inspecting the 3-by-3 window around the current cell. The SFD method
95 assigns a unique flow direction towards the steepest downslope neigh‐
96 bor. The MFD method assigns multiple flow directions towards all downs‐
97 lope neighbors.
98 Flow direction to steepest downslope neighbor (SFD). Flow direction to all downslope neighbors (MFD).
102 The SFD and the MFD method cannot compute flow directions for cells
105 which have the same height as all their neighbors (flat areas) or cells
106 which do not have downslope neighbors (one-cell pits).
107 · On plateaus (flat areas that spill out) r.terraflow routes flow
109 so that globally the flow goes towards the spill cells of the
110 plateaus.
111 · On sinks (flat areas that do not spill out, including one-cell
113 pits) r.terraflow assigns flow by flooding the terrain until
114 all the sinks are filled and assigning flow directions on the
115 filled terrain.
116 In order to flood the terrain, r.terraflow identifies all sinks and
118 partitions the terrain into sink-watersheds (a sink-watershed contains
119 all the cells that flow into that sink), builds a graph representing
120 the adjacency information of the sink-watersheds, and uses this
121 sink-watershed graph to merge watersheds into each other along their
122 lowest common boundary until all watersheds have a flow path outside
123 the terrain. Flooding produces a sink-less terrain in which every cell
124 has a downslope flow path leading outside the terrain and therefore
125 every cell in the terrain can be assigned SFD/MFD flow directions as
126 above.
127 Once flow directions are computed for every cell in the terrain, r.ter‐
129 raflow computes flow accumulation by routing water using the flow
130 directions and keeping track of how much water flows through each cell.
131 If flow accumulation of a cell is larger than the value given by the
133 d8cut option, then the flow of this cell is routed to its neighbors
134 using the SFD (D8) model. This option affects only the flow accumula‐
135 tion raster and is meaningful only for MFD flow (i.e. if the -s flag is
136 not used); If this option is used for SFD flow it is ignored. The
137 default value of d8cut is infinity.
138 r.terraflow also computes the tci raster (topographic convergence
140 index, defined as the logarithm of the ratio of flow accumulation and
141 local slope).
142 For more details on the algorithms see [1,2,3] below.
144
146 One of the techniques used by r.terraflow is the space-time trade-off.
147 In particular, in order to avoid searches, which are I/O-expensive,
148 r.terraflow computes and works with an augmented elevation raster in
149 which each cell stores relevant information about its 8 neighbors, in
150 total up to 80B per cell. As a result r.terraflow works with interme‐
151 diate temporary files that may be up to 80N bytes, where N is the num‐
152 ber of cells (rows x columns) in the elevation raster (more precisely,
153 80K bytes, where K is the number of valid (not no-data) cells in the
154 input elevation raster).
155 All these intermediate temporary files are stored in the path specified
157 by the directory option. Note: directory must contain enough free disk
158 space in order to store up to 2 x 80N bytes.
159 The memory option can be used to set the maximum amount of main memory
161 (RAM) the module will use during processing. In practice its value
162 should be an underestimate of the amount of available (free) main mem‐
163 ory on the machine. r.terraflow will use at all times at most this much
164 memory, and the virtual memory system (swap space) will never be used.
165 The default value is 300 MB.
166 The stats option defines the name of the file that contains the statis‐
168 tics (stats) of the run.
169 r.terraflow has a limit on the number of rows and columns (max 32,767
171 each).
172 The internal type used by r.terraflow to store elevations can be
174 defined at compile-time. By default, r.terraflow is compiled to store
175 elevations internally as floats. Other versions can be created by the
176 user if needed.
177 Hints concerning compilation with storage of elevations internally as
179 shorts: such a version uses less space (up to 60B per cell, up to 60N
180 intermediate file) and therefore is more space and time efficient.
181 r.terraflow is intended for use with floating point raster data
182 (FCELL), and r.terraflow (short) with integer raster data (CELL) in
183 which the maximum elevation does not exceed the value of a short
184 SHRT_MAX=32767 (this is not a constraint for any terrain data of the
185 Earth, if elevation is stored in meters). Both r.terraflow and r.ter‐
186 raflow (short) work with input elevation rasters which can be either
187 integer, floating point or double (CELL, FCELL, DCELL). If the input
188 raster contains a value that exceeds the allowed internal range (short
189 for r.terraflow (short), float for r.terraflow), the program exits with
190 a warning message. Otherwise, if all values in the input elevation
191 raster are in range, they will be converted (truncated) to the internal
192 elevation type (short for r.terraflow (short), float for r.terraflow).
193 In this case precision may be lost and artificial flat areas may be
194 created. For instance, if r.terraflow (short) is used with floating
195 point raster data (FCELL or DCELL), the values of the elevation will be
196 truncated as shorts. This may create artificial flat areas, and the
197 output of r.terraflow (short) may be less realistic than those of
198 r.terraflow on floating point raster data. The outputs of r.terraflow
199 (short) and r.terraflow are identical for integer raster data (CELL
200 maps).
201
203 Example for small area in North Carolina sample dataset to calculate
204 flow accumulation:
205 g.region raster=elev_lid792_1m
206 r.terraflow elevation=elev_lid792_1m accumulation=elev_lid792_1m_accumulation
207 Flow accumulation
208 Spearfish sample data set:
210 g.region raster=elevation.10m -p
211 r.terraflow elev=elevation.10m filled=elevation10m.filled \
212 dir=elevation10m.mfdir swatershed=elevation10m.watershed \
213 accumulation=elevation10m.accu tci=elevation10m.tci
214 g.region raster=elevation.10m -p
215 r.terraflow elev=elevation.10m filled=elevation10m.filled \
216 dir=elevation10m.mfdir swatershed=elevation10m.watershed \
217 accumulation=elevation10m.accu tci=elevation10m.tci d8cut=500 memory=800 \
218 stats=elevation10mstats.txt
219
221 1 The TerraFlow project at Duke University
222 2 I/O-efficient algorithms for problems on grid-based terrains.
224 Lars Arge, Laura Toma, and Jeffrey S. Vitter. In Proc. Workshop
225 on Algorithm Engineering and Experimentation, 2000. To appear in
226 Journal of Experimental Algorithms.
227 3 Flow computation on massive grids. Lars Arge, Jeffrey S. Chase,
229 Patrick N. Halpin, Laura Toma, Jeffrey S. Vitter, Dean Urban and
230 Rajiv Wickremesinghe. In Proc. ACM Symposium on Advances in Geo‐
231 graphic Information Systems, 2001.
232 4 Flow computation on massive grid terrains. Lars Arge, Jeffrey
234 S. Chase, Patrick N. Halpin, Laura Toma, Jeffrey S. Vitter, Dean
235 Urban and Rajiv Wickremesinghe. In GeoInformatica, Interna‐
236 tional Journal on Advances of Computer Science for Geographic
237 Information Systems, 7(4):283-313, December 2003.
238
240 r.flow, r.basins.fill, r.drain, r.topidx, r.topmodel, r.water.outlet,
241 r.watershed
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244 Original version of program: The TerraFlow
245 project, 1999, Duke University.
246 Lars Arge, Jeff Chase, Pat Halpin, Laura Toma, Dean Urban, Jeff
247 Vitter, Rajiv Wickremesinghe.
248 Porting to GRASS GIS, 2002:
250 Lars Arge, Helena Mitasova, Laura Toma.
251 Contact: Laura Toma
253 Last changed: $Date: 2017-02-11 22:22:16 +0100 (Sat, 11 Feb 2017) $
255
257 Available at: r.terraflow source code (history)
258 Main index | Raster index | Topics index | Keywords index | Graphical
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261 © 2003-2019 GRASS Development Team, GRASS GIS 7.6.0 Reference Manual
263GRASS 7.6.0 r.terraflow(1)