1r.flow(1) Grass User's Manual r.flow(1)
2
6 r.flow - Construction of slope curves (flowlines), flowpath lengths,
7 and flowline densities (upslope areas) from a raster digital elevation
8 model (DEM)
9
11 raster
12
14 r.flow
15 r.flow help
16 r.flow [-u3mqh] elevin=name [aspin=name] [barin=name] [skip=inte‐
17 ger] [bound=integer] [flout=name] [lgout=name] [dsout=name]
18 Flags:
20 -u Compute upslope flowlines instead of default downhill flowlines
21 -3 3-D lengths instead of 2-D
23 -m Use less memory, at a performance penalty
25 -q Quiet operation
27 -h Display Reference Information
29 Parameters:
31 elevin=name
32 Input elevation file
33 aspin=name
35 Input aspect file
36 barin=name
38 Input barrier file
39 skip=integer
41 Number of cells between flowlines Options: 1-360 Default: 7
42 bound=integer
44 Maximum number of segments per flowline Options: 0-1609 Default:
45 1609
46 flout=name
48 Output flowline vector file
49 lgout=name
51 Output flowpath length raster file
52 dsout=name
54 Output flowline density raster file
55
57 This program generates flowlines using a combined raster-vector
58 approach (see Mitasova and Hofierka 1993 and Mitasova et al. 1995))
59 from an input elevation raster map elevin (integer or floating point),
60 and optionally an input aspect raster map aspin and/or an input barrier
61 raster map barin. There are three possible output maps which can be
62 produced in any combination simultaneously: a vector file flout of
63 flowlines, a raster map lgout of flowpath lengths, and a raster map
64 dsout of flowline densities (which are equal upslope contributed areas
65 per unit width, when multiplied by resolution).
66 Aspect used for input must follow the same rules as aspect computed in
68 other GRASS programs (see v.surf.rst or r.slope.aspect).
69 Flowline output is given in a vector map flout, (flowlines generated
71 downhill). The line segments of flowline vectors have endpoints on
72 edges of a grid formed by drawing imaginary lines through the centers
73 of the cells in the elevation map. Flowlines are generated from each
74 cell downhill by default; they can be generated uphill using the flag
75 -u. A flowline stops if its next segment would reverse the direction of
76 flow (from up to down or vice-versa), cross a barrier, or arrive at a
77 cell with undefined elevation or aspect. Another option, skip=val,
78 indicates that only the flowlines from every val-th cell are to be
79 included in flout. The default skip is max(1, /50, /50). A high skip
80 usually speeds up processing time and often improves the readability of
81 a visualization of flout.
82 Flowpath length output is given in a raster map lgout. The value in
84 each grid cell is the sum of the planar lengths of all segments of the
85 flowline generated from that cell. If the flag -3 is given, elevation
86 is taken into account in calculating the length of each segment.
87 Flowline density downhill or uphill output is given in a raster map
89 dsout. The value in each grid cell is the number of flowlines which
90 pass through that grid cell, that means the number of flowlines from
91 the entire map which have segment endpoints within that cell. With the
92 -m flag less memory is used as aspect at each cell is computed on the
93 fly. This option incurs a severe performance penalty. If this flag is
94 given, the aspect input map (if any) will be ignored. The barin param‐
95 eter is a raster file name with non-zero values representing barriers
96 as input.
97
99 For best results, use input elevation maps with high precision units
100 (e.g., centimeters) so that flowlines do not terminate prematurely in
101 flat areas. To prevent the creation of tiny flowline segments with
102 imperceivable changes in elevation, an endpoint which would land very
103 close to the center of a grid cell is quantized to the exact center of
104 that cell. The maximum distance between the intercepts along each axis
105 of a single diagonal segment and another segment of 1/2 degree differ‐
106 ent aspect is taken to be "very close" for that axis. Note that this
107 distance (the so-called "quantization error") is about 1-2% of the res‐
108 olution on maps with square cells.
109 The values in length maps computed using the -u flag represent the dis‐
111 tances from each cell to an upland flat or singular point. Such dis‐
112 tances are useful in water erosion modeling for computation of the LS
113 factor in the standard form of USLE. Uphill flowlines merge on ridge
114 lines; by redirecting the order of the flowline points in the output
115 vector map, dispersed waterflow can be simulated. The density map can
116 be used for the extraction of ridge lines.
117 Computing the flowlines downhill simulates the actual flow (also known
119 as the raindrop method). These flowlines tend to merge in valleys; they
120 can be used for localization of areas with waterflow accumulation and
121 for the extraction of channels. The downslope flowline density multi‐
122 plied by the resolution can be used as an approximation of the upslope
123 contributing area per unit contour width. This area is a measure of
124 potential water flux for the steady state conditions and can be used in
125 the modeling of water erosion for the computation of the unit stream
126 power based LS factor or sediment transport capacity.
127 The program has been designed for modeling erosion on hillslopes and
129 has rather strict conditions for ending flowlines. It is therefore not
130 very suitable for the extraction of stream networks or delineation of
131 watersheds unless a DEM without pits or flat areas is available
132 (r.fill.dir can be used to fill pits).
133 To label the vector flowlines automatically, the user can use v.cate‐
135 gory (add categories).
136
138 1. Construction of flow-lines (slope-lines): r.flow uses an original
139 vector-grid algorithm which uses an infinite number of directions
140 between 0.0000... and 360.0000... and traces the flow as a line (vec‐
141 tor) in the direction of gradient (rather than from cell to cell in one
142 of the 8 directions = D-infinity algorithm). They are traced in any
143 direction using aspect (so there is no limitation to 8 directions
144 here). The D8 algorithm produces zig-zag lines. The value in the outlet
145 is very similar for both r.flow and r.flowmd (GRASS 5 only) algorithms
146 (because it is essentially the watershed area), however the spatial
147 distribution of flow, especially on hillslopes is quite different. It
148 is still a 1D flow routing so the dispersal flow is not accurately
149 described, but still better than D8.
150 2. Computation of contributing areas: r.flow uses a single flow algo‐
152 rithm, i.e. all flow is transported to a single cell downslope.
153
155 Differences between r.flow and r.flowmd
156 1
158 r.flow has an option to compute slope and aspect internally
159 thus making the program capable to process much larger data sets
160 than r.flowmd. It has also 2 additional options for handling of
161 large data sets but it is not known that they work properly.
162 2
164 the programs handle the special cases when the flowline passes
165 exactly (or very close) through the grid vertices differently.
166 3
168 r.flowmd has the simplified multiple flow addition so the
169 results are smoother.
170 In conclusion, r.flowmd produces nicer results but is slower and it
172 does not support as large data sets as r.flow.
173
175 "ERROR: r.flow: " input " file's resolution differs from current"
176 region resolution
177 The resolutions of all input files and the current region must match.
179 "ERROR: r.flow: resolution too unbalanced (" val " x " val ")" The dif‐
181 ference in length between the two axes of a grid cell is so great that
182 quantization error is larger than one of the dimensions. Resample the
183 map and try again.
184
186 r.basins.fill, r.drain, r.fill.dir, r.water.outlet, r.watershed, v.cat‐
187 egory, v.to.rast
188
190 Original version of program:
191 Maros Zlocha and Jaroslav Hofierka, Comenius University, Bratislava,
192 Slovakia,
193 The current version of the program (adapted for GRASS5.0):
195 Joshua Caplan, Mark Ruesink, Helena Mitasova, University of Illinois at
196 Urbana-Champaign with support from USA CERL.
197 GMSL/University of Illinois at Urbana-Champaign
198
200 Mitasova, H., L. Mitas, 1993, Interpolation by regularized spline with
201 tension : I. Theory and implementation. Mathematical Geology 25, p.
202 641-655. (online)
203 Mitasova and Hofierka 1993 : Interpolation by Regularized Spline with
205 Tension: II. Application to Terrain Modeling and Surface Geometry Anal‐
206 ysis. Mathematical Geology 25(6), 657-669. (online)
207 Mitasova, H., Mitas, L., Brown, W.M., Gerdes, D.P., Kosinovsky, I.,
209 Baker, T., 1995: Modeling spatially and temporally distributed phenom‐
210 ena: New methods and tools for GRASS GIS. International Journal of Geo‐
211 graphical Information Systems 9(4), 433-446.
212 Mitasova, H., J. Hofierka, M. Zlocha, L.R. Iverson, 1996, Modeling
214 topographic potential for erosion and deposition using GIS. Int. Jour‐
215 nal of Geographical Information Science, 10(5), 629-641. (reply to a
216 comment to this paper appears in 1997 in Int. Journal of Geographical
217 Information Science, Vol. 11, No. 6)
218 Mitasova, H.(1993): Surfaces and modeling. Grassclippings (winter and
220 spring) p.18-19.
221 Last changed: $Date: 2006/04/20 21:31:23 $
223 Full index
225GRASS 6.2.2 r.flow(1)