r.flow(1grass) GRASS GIS User's Manual r.flow(1grass)
NAME
r.flow - Constructs flowlines.
Computes flowlines, flowpath lengths, and flowaccumulation (contribut-
ing areas) from a elevation raster map.
KEYWORDS
raster, hydrology
SYNOPSIS
r.flow
r.flow --help
r.flow [-u3m] elevation=name [aspect=name] [barrier=name]
[skip=integer] [bound=integer] [flowline=name] [flowlength=name]
[flowaccumulation=name] [--overwrite] [--help] [--verbose]
[--quiet] [--ui]
Flags:
-u
Compute upslope flowlines instead of default downhill flowlines
-3
3D lengths instead of 2D
-m
Use less memory, at a performance penalty
--overwrite
Allow output files to overwrite existing files
--help
Print usage summary
--verbose
Verbose module output
--quiet
Quiet module output
--ui
Force launching GUI dialog
Parameters:
elevation=name [required]
Name of input elevation raster map
aspect=name
Name of input aspect raster map
barrier=name
Name of input barrier raster map
skip=integer
Number of cells between flowlines
bound=integer
Maximum number of segments per flowline
flowline=name
Name for output flow line vector map
flowlength=name
Name for output flow path length raster map
flowaccumulation=name
Name for output flow accumulation raster map
DESCRIPTION
r.flow generates flowlines using a combined raster-vector approach (see
Mitasova and Hofierka 1993 and Mitasova et al. 1995) from an input ele-
vation raster map (integer or floating point), and optionally an input
aspect raster map and/or an input barrier raster map.
There are three possible output raster maps which can be produced in
any combination simultaneously: a vector map flowline of flowlines, a
raster map flowlength of flowpath lengths, and a raster map flowaccumu-
lation of flowline densities (which are equal upslope contributed areas
per unit width, when multiplied by resolution).
NOTES
Aspect used for input must follow the same rules as aspect computed in
other modules (see v.surf.rst or r.slope.aspect).
Output flowline is generated downhill. The line segments of flowline
vectors have endpoints on edges of a grid formed by drawing imaginary
lines through the centers of the cells in the elevation map. Flowlines
are generated from each cell downhill by default; they can be generated
uphill using the flag -u. A flowline stops if its next segment would
reverse the direction of flow (from up to down or vice-versa), cross a
barrier, or arrive at a cell with undefined elevation or aspect. An-
other option, skip, indicates that only the flowlines from every val-th
cell are to be included in flowline. The default skip is max(1, <rows
in elevation>/50, <cols in elevation>/50). A high skip usually speeds
up processing time and often improves the readability of a visualiza-
tion of flowline.
Flowpath length output is given in a raster map flowlength. The value
in each grid cell is the sum of the planar lengths of all segments of
the flowline generated from that cell. If the flag -3 is given, eleva-
tion is taken into account in calculating the length of each segment.
Flowline density downhill or uphill output is given in a raster map
flowaccumulation. The value in each grid cell is the number of flow-
lines which pass through that grid cell, that means the number of flow-
lines from the entire map which have segment endpoints within that
cell. With the -m flag less memory is used as aspect at each cell is
computed on the fly. This option incurs a severe performance penalty.
If this flag is given, the aspect input map (if any) will be ignored.
The barrier parameter is a raster map name with non-zero values repre-
senting barriers as input.
For best results, use input elevation maps with high precision units
(e.g., centimeters) so that flowlines do not terminate prematurely in
flat areas. To prevent the creation of tiny flowline segments with im-
perceivable changes in elevation, an endpoint which would land very
close to the center of a grid cell is quantized to the exact center of
that cell. The maximum distance between the intercepts along each axis
of a single diagonal segment and another segment of 1/2 degree differ-
ent aspect is taken to be "very close" for that axis. Note that this
distance (the so-called "quantization error") is about 1-2% of the res-
olution on maps with square cells.
The values in length maps computed using the -u flag represent the dis-
tances from each cell to an upland flat or singular point. Such dis-
tances are useful in water erosion modeling for computation of the LS
factor in the standard form of USLE. Uphill flowlines merge on ridge
lines; by redirecting the order of the flowline points in the output
vector map, dispersed waterflow can be simulated. The density map can
be used for the extraction of ridge lines.
Computing the flowlines downhill simulates the actual flow (also known
as the raindrop method). These flowlines tend to merge in valleys; they
can be used for localization of areas with waterflow accumulation and
for the extraction of channels. The downslope flowline density multi-
plied by the resolution can be used as an approximation of the upslope
contributing area per unit contour width. This area is a measure of po-
tential water flux for the steady state conditions and can be used in
the modeling of water erosion for the computation of the unit stream
power based LS factor or sediment transport capacity.
r.flow has been designed for modeling erosion on hillslopes and has
rather strict conditions for ending flowlines. It is therefore not very
suitable for the extraction of stream networks or delineation of water-
sheds unless a DEM without pits or flat areas is available (r.fill.dir
can be used to fill pits).
To label the vector flowlines automatically, the user can use v.cate-
gory (add categories).
Algorithm background
r.flow uses an original vector-grid algorithm which uses an infinite
number of directions between 0.0000... and 360.0000... and traces the
flow as a line (vector) in the direction of gradient (rather than from
cell to cell in one of the 8 directions = D-infinity algorithm). They
are traced in any direction using aspect (so there is no limitation to
8 directions here). The D8 algorithm produces zig-zag lines. The value
in the outlet is very similar for r.flow algorithm (because it is es-
sentially the watershed area), however the spatial distribution of
flow, especially on hillslopes is quite different. It is still a 1D
flow routing so the dispersal flow is not accurately described, but
still better than D8.
r.flow uses a single flow algorithm, i.e. all flow is transported to a
single cell downslope.
Diagnostics
Elevation raster map resolution differs from current region resolution
The resolutions of all input raster maps and the current region must
match (see g.region).
Resolution too unbalanced
The difference in length between the two axes of a grid cell is so
great that quantization error is larger than one of the dimensions. Re-
sample the map and try again.
EXAMPLE
In this example a flow line vector map, a flow path length raster map
and a flow accumulation raster map are computed from an elevation
raster map (North Carolina sample dataset):
g.region raster=elevation -p
r.flow elevation=elevation skip=3 flowline=flowline flowlength=flowlength \
flowaccumulation=flowaccumulation
Figure: Flow lines with underlying elevation map; flow lines with un-
derlying flow path lengths (in map units: meters); flow accumulation
map (zoomed view)
REFERENCES
• Mitasova, H., L. Mitas, 1993, Interpolation by regularized
spline with tension : I. Theory and implementation. Mathemati-
cal Geology 25, p. 641-655. (online)
• Mitasova and Hofierka 1993 : Interpolation by Regularized
Spline with Tension: II. Application to Terrain Modeling and
Surface Geometry Analysis. Mathematical Geology 25(6), 657-669
(online).
• Mitasova, H., Mitas, L., Brown, W.M., Gerdes, D.P., Kosinovsky,
I., Baker, T., 1995: Modeling spatially and temporally distrib-
uted phenomena: New methods and tools for GRASS GIS. Interna-
tional Journal of Geographical Information Systems 9(4),
433-446.
• Mitasova, H., J. Hofierka, M. Zlocha, L.R. Iverson, 1996, Mod-
eling topographic potential for erosion and deposition using
GIS. Int. Journal of Geographical Information Science, 10(5),
629-641. (reply to a comment to this paper appears in 1997 in
Int. Journal of Geographical Information Science, Vol. 11, No.
6)
• Mitasova, H.(1993): Surfaces and modeling. Grassclippings (win-
ter and spring) p.18-19.
SEE ALSO
r.basins.fill, r.drain, r.fill.dir, r.water.outlet, r.watershed,
v.category, v.to.rast
AUTHORS
Original version of program: Maros Zlocha and Jaroslav Hofierka, Come-
nius University, Bratislava, Slovakia
The current version of the program (adapted for GRASS 5.0): Joshua Ca-
plan, Mark Ruesink, Helena Mitasova, University of Illinois at Ur-
bana-Champaign with support from USA CERL. GMSL/University of Illinois
at Urbana-Champaign
SOURCE CODE
Available at: r.flow source code (history)
Accessed: unknown
Main index | Raster index | Topics index | Keywords index | Graphical
index | Full index
© 2003-2022 GRASS Development Team, GRASS GIS 7.8.7 Reference Manual
GRASS 7.8.7 r.flow(1grass)
Generated by dwww version 1.14 on Sun Dec 29 18:57:29 CET 2024.