Ubuntu Manpage: r.terraflow - Performs flow computation for massive grids.

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Provided by: grass-doc_7.0.3-1build1_all

NAME

       r.terraflow  - Performs flow computation for massive grids.
       Float version.

KEYWORDS

       raster, hydrology, flow, accumulation, sink

SYNOPSIS

       r.terraflow
       r.terraflow --help
       r.terraflow  [-s]  elevation=name  filled=name  direction=name swatershed=name accumulation=name tci=name
       [d8cut=float]    [memory=integer]     [directory=string]     [stats=string]     [--overwrite]    [--help]
       [--verbose]  [--quiet]  [--ui]

   Flags:
       -s
           SFD (D8) flow (default is MFD)
           SFD: single flow direction, MFD: multiple flow direction

       --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

       filled=name [required]
           Name for output filled (flooded) elevation raster map

       direction=name [required]
           Name for output flow direction raster map

       swatershed=name [required]
           Name for output sink-watershed raster map

       accumulation=name [required]
           Name for output flow accumulation raster map

       tci=name [required]
           Name for output topographic convergence index (tci) raster map

       d8cut=float
           Routing using SFD (D8) direction
           If  flow  accumulation  is  larger than this value it is routed using SFD (D8) direction (meaningfull
           only for MFD flow). If no answer is given it defaults to infinity.

       memory=integer
           Maximum memory to be used (in MB)
           Default: 300

       directory=string
           Directory to hold temporary files (they can be large)

       stats=string
           Name of file containing runtime statistics

DESCRIPTION

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 maps.

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.

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.

If flow accumulation of a cell is larger than the value given by the d8cut option, 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.

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] below.

NOTES

       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 no-data) cells in the input elevation raster).

       All  these  intermediate temporary files are stored in the path specified by the STREAM_DIR option. Note:
       STREAM_DIR must contain enough free disk space in order to store up to 2 x 80N bytes.

       The memory option can be used to set the maximum amount of main memory (RAM) the module will  use  during
       processing.  In  practice  its  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 (swap space) will never be used. The default value is 300 MB.

       The stats option defines the name of the file that contains the statistics (stats) of the run.

       r.terraflow has a limit on the number of rows and columns (max 32,767 each).

       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. Other versions can be  created  by  the
       user if needed.

       Hints concerning compilation with storage of elevations internally as shorts:
       such  a  version  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  output  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 for integer raster data (CELL maps).

EXAMPLES

       Example for small area in North Carolina sample dataset:
       g.region raster=elev_lid792_1m
       r.terraflow elevation=elev_lid792_1m filled=elev_lid792_1m_filled \
           direction=elev_lid792_1m_direction swatershed=elev_lid792_1m_swatershed \
           accumulation=elev_lid792_1m_accumulation tci=elev_lid792_1m_tci
       Flow accumulation

       Spearfish sample data set:
       g.region raster=elevation.10m -p
       r.terraflow elev=elevation.10m filled=elevation10m.filled \
           dir=elevation10m.mfdir swatershed=elevation10m.watershed \
           accumulation=elevation10m.accu tci=elevation10m.tci
       g.region raster=elevation.10m -p
       r.terraflow elev=elevation.10m filled=elevation10m.filled \
           dir=elevation10m.mfdir swatershed=elevation10m.watershed \
           accumulation=elevation10m.accu tci=elevation10m.tci d8cut=500 memory=800 \
           stats=elevation10mstats.txt

AUTHORS

       Original version of program: The                             TerraFlow project, 1999, Duke University.
           Lars Arge, Jeff Chase, Pat Halpin, Laura Toma, Dean Urban, Jeff Vitter, Rajiv Wickremesinghe.

       Porting to GRASS GIS, 2002:
           Lars Arge, Helena Mitasova, Laura Toma.

       Contact:  Laura Toma

REFERENCES

       1      The TerraFlow project at Duke University

       2      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.

       3      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.

       4      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: 2015-01-14 09:26:47 +0100 (Wed, 14 Jan 2015) $

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