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r.terraflow - computation of flow direction, flow accumulation and
other basic topographic terrain indices from a raster digital elevation
model (DEM).
(GRASS Raster Program)
SYNOPSIS
r.terraflow
r.terraflow help
r.terraflow [ -sq ] elev=name filled=name direction=name
swatershed=name accumulation=name tci=name [d8cut=value] [memory=value]
[STREAM_DIR=name] [stats=name]
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r.terraflow takes as input a raster digital elevation model (DEM) and
computes the flow direction raster and the flow accumulation raster, as
well as the flooded elevation raster, sink-watershed raster (partition
into watersheds around sinks) and tci (topographic convergence index)
raster.
r.terraflow computes these rasters using well-known approaches, with
the difference that its emphasis is on the computational complexity of
the algorithms, rather than on modeling realistic flow. r.terraflow
emerged from the necessity of having scalable software able to process
efficiently very large terrains. It is based on theoretically optimal
algorithms developed in the framework of I/O-efficient algorithms.
r.terraflow was designed and optimized especially for massive grids and
is able to process terrains which were impractical with similar
functions existing in other GIS systems.
Flow directions are computed using either the MFD (Multiple Flow
Direction) model or the SFD (Single Flow Direction, or D8) model,
illustrated below. Both methods compute downslope flow directions by
inspecting the 3-by-3 window around the current cell. The SFD method
assigns a unique flow direction towards the steepest downslope
neighbor. The MFD method assigns multiple flow directions towards all
downslope neighbors.
[SFD] [MFD]
Flow direction to steepest
downslope neighbor (SFD). Flow direction to all
downslope neighbors (MFD).
The SFD and the MFD method cannot compute flow directions for cells
which have the same height as all their neighbors (flat areas) or cells
which do not have downslope neighbors (one-cell pits).
* On plateaus (flat areas that spill out) r.terraflow routes flow so
that globally the flow goes towards the spill cells of the
plateaus.
* On sinks (flat areas that do not spill out, including one-cell
pits) r.terraflow assigns flow by flooding the terrain until all
the sinks are filled and assigning flow directions on the filled
terrain.
In order to flood the terrain, r.terraflow identifies all sinks and
partitions the terrain into sink-watersheds (a sink-watershed contains
all the cells that flow into that sink), builds a graph representing
the adjacency information of the sink-watersheds, and uses this
sink-watershed graph to merge watersheds into each other along their
lowest common boundary until all watersheds have a flow path outside
the terrain. Flooding produces a sink-less terrain in which every cell
has a downslope flow path leading outside the terrain and therefore
every cell in the terrain can be assigned SFD/MFD flow directions as
above.
Once flow directions are computed for every cell in the terrain,
r.terraflow computes flow accumulation by routing water using the flow
directions and keeping track of how much water flows through each cell.
r.terraflow also computes the tci raster (topographic convergence
index, defined as the logarithm of the ratio of flow accumulation and
local slope).
For more details on the algorithms see [1,2,3].
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The program will run non-interactively if the user specifies program
arguments and flag settings on the command line using the following
form:
r.terraflow [ -sq ] elev=name filled=name direction=name
swatershed=name accumulation=name tci=name [d8cut=value] [memory=value]
[STREAM_DIR=name] [stats=name]
Alternatively, the user can simply type r.terraflow on the command line
and the program will ask for parameter values and flag settings
interactively, using the standard GRASS parser interface.
Flags:
-s
Use SFD (D8) flow. By default MFD flow is used.
-q
Run quietly (do not display status messages). By default
r.terraflow is run verbosely.
Parameters:
elev=name
Input elevation raster. Required.
filled=name
Output filled (flooded) elevation raster. Required.
direction =name
Output flow direction raster. Required.
swatershed =name
Output sink-watershed raster. Required.
accumulation =name
Output flow accumulation raster. Required.
tci =name
Output topographic convergence index (tci) raster. Required.
[d8cut =value]
If flow accumulation of a cell is larger than this value, then
the flow of this cell is routed to its neighbors using the SFD
(D8) model. This option affects only the flow accumulation
raster and is meaningful only for MFD flow (i.e. if the -s flag
is not used); If this option is used for SFD flow it is ignored.
The default value of d8cut is infinity.
[memory =value (in MB)]
The main memory size (in MB) to be used by r.terraflow. In
practice value should be an underestimate of the amount of
available (free) main memory on the machine. r.terraflow will
use at all times at most this much memory, and the virtual
memory system will never be in use. The default value is 300 MB.
[STREAM_DIR =path name]
Location of the intermediate files generated by r.terraflow. The
default location is /var/tmp.
[stats =name]
The name of the file that contains the statistics (stats) of the
run. The default name is stats.out (in the current directory).
Examples
* r.terraflow elev=spearfish filled=spearfish-filled
dir=spearfish-mfdir swatershed=spearfish-watershed
accumulation=spearfish-accu tci=spearfish-tci
* r.terraflow elev=spearfish filled=spearfish-filled
dir=spearfish-mfdir swatershed=spearfish-watershed
accumulation=spearfish-accu tci=spearfish-tci d8cut=500 memory=800
STREAM-DIR=/var/tmp/ stats=spearfish-stats.txt
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One of the techniques used by r.terraflow is the space-time trade-off.
In particular, in order to avoid searches, which are I/O-expensive,
r.terraflow computes and works with an augmented elevation raster in
which each cell stores relevant information about its 8 neighbors, in
total up to 80B per cell. As a result r.terraflow works with
intermediate temporary files that may be up to 80N bytes, where N is
the number of cells (rows x columns) in the elevation raster (more
precisely, 80K bytes, where K is the number of valid (not nodata) cells
in the input elevation raster). All this intermediate temporary files
are stored in the path specified by STREAM_DIR. Note: STREAM_DIR must
contain enough free disk space in order to store up to 2 x 80N bytes.
The internal type used by r.terraflow to store elevations can be
defined at compile-time. By default, r.terraflow is compiled to store
elevations internally as floats. A version which is compiled to store
elevations internally as shorts is available as r.terraflow.short.
Other versions can be created by the user if needed.
r.terraflow.short uses less space (up to 60B per cell, up to 60N
intermediate file) and therefore is more space and time efficient.
r.terraflow is intended for use with floating point raster data
(FCELL), and r.terraflow.short with integer raster data (CELL) in which
the maximum elevation does not exceed the value of a short
SHRT_MAX=32767 (this is not a constraint for any terrain data of the
Earth, if elevation is stored in meters).
Both r.terraflow and r.terraflow.short work with input elevation
rasters which can be either integer, floating point or double (CELL,
FCELL, DCELL). If the input raster contains a value that exceeds the
allowed internal range (short for r.terraflow.short, float for
r.terraflow), the program exits with a warning message. Otherwise, if
all values in the input elevation raster are in range, they will be
converted (truncated) to the internal elevation type (short for
r.terraflow.short, float for r.terraflow). In this case precision may
be lost and artificial flat areas may be created.
For instance, if r.terraflow.short is used with floating point raster
data (FCELL or DCELL), the values of the elevation will be truncated as
shorts. This may create artificial flat areas, and the outpus of
r.terraflow.short may be less realistic than those of r.terraflow on
floating point raster data. The outputs of r.terraflow.short and
r.terraflow are identical on integer raster data (CELL).
СÐ. ТÐÐÐÐ
* The [1]TerraFlow project at Duke University
* [2]r.flow, [3]r.basins.fill, [4]r.drain, [5]r.topmodel,
[6]r.water.outlet, [7]r.watershed
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Original version of program: The [8]TerraFlow project, 1999, Duke
University.
[9]Lars Arge, [10]Jeff Chase, [11]Pat Halpin, [12]Laura Toma,
[13]Dean Urban, [14]Jeff Vitter, Rajiv Wickremesinghe.
Porting for GRASS, 2002:
[15]Lars Arge, [16]Helena Mitasova, [17]Laura Toma.
Contact: [18]Laura Toma
ССЫÐÐÐ
1. [19]I/O-efficient algorithms for problems on grid-based terrains.
Lars Arge, Laura Toma, and Jeffrey S. Vitter. In Proc. Workshop on
Algorithm Engineering and Experimentation, 2000. To appear in
Journal of Experimental Algorithms.
2. [20]Flow computation on massive grids. Lars Arge, Jeffrey S. Chase,
Patrick N. Halpin, Laura Toma, Jeffrey S. Vitter, Dean Urban and
Rajiv Wickremesinghe. In Proc. ACM Symposium on Advances in
Geographic Information Systems, 2001.
3. [21]Flow computation on massive grid terrains. Lars Arge, Jeffrey
S. Chase, Patrick N. Halpin, Laura Toma, Jeffrey S. Vitter, Dean
Urban and Rajiv Wickremesinghe. In GeoInformatica, International
Journal on Advances of Computer Science for Geographic Information
Systems, 7(4):283-313, December 2003.
Last changed: $Date: 2008-08-19 16:28:59 -0500 (ÐÑ, 19 авг 2008) $
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References
1. http://www.cs.duke.edu/geo*/terraflow/
2. file://localhost/root/tmp/2/fin/r.flow.html
3. file://localhost/root/tmp/2/fin/r.basins.fill.html
4. file://localhost/root/tmp/2/fin/r.drain.html
5. file://localhost/root/tmp/2/fin/r.topidx.html>r.topidx</a>,<ahref=
6. file://localhost/root/tmp/2/fin/r.water.outlet.html
7. file://localhost/root/tmp/2/fin/r.watershed.html
8. http://www.cs.duke.edu/geo*/terraflow/
9. http://www.daimi.au.dk/~large/
10. http://www.cs.duke.edu/~chase/
11. http://www.env.duke.edu/faculty/bios/halpin.html
12. http://www.bowdoin.edu/~ltoma/
13. http://www.env.duke.edu/faculty/bios/urban.html
14. http://www.science.purdue.edu/jsv/
15. http://www.daimi.au.dk/~large/
16. http://skagit.meas.ncsu.edu/~helena/index.html
17. http://www.bowdoin.edu/~ltoma/
18. mailto:ltoma@bowdoin.edu
19. http://www.cs.duke.edu/geo*/terraflow/papers/alenex00_drainage.ps.gz
20. http://www.cs.duke.edu/geo*/terraflow/papers/acmgis01_terraflow.pdf
21. http://www.cs.duke.edu/geo*/terraflow/papers/journal_terraflow.pdf