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       r.terraflow  - Performs flow computation for massive grids.


       raster, hydrology, flow, accumulation, sink


       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]

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

           Allow output files to overwrite existing files

           Print usage summary

           Verbose module output

           Quiet module output

           Force launching GUI dialog

       elevation=name [required]
           Name of input elevation raster map

           Name for output filled (flooded) elevation raster map

           Name for output flow direction raster map

           Name for output sink-watershed raster map

           Name for output flow accumulation raster map

           Name for output topographic convergence index (tci) raster map

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

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

           Directory to hold temporary files (they can be large)

           Name for output file containing runtime statistics


       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

       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.


       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  directory
       option.  Note:  directory  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

       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).


       Example for small area in North Carolina sample dataset to calculate flow accumulation:
       g.region raster=elev_lid792_1m
       r.terraflow elevation=elev_lid792_1m accumulation=elev_lid792_1m_accumulation
       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 \
       g.region raster=elevation.10m -p
       r.terraflow elev=elevation.10m filled=elevation10m.filled \
           dir=elevation10m.mfdir swatershed=elevation10m.watershed \
           accumulation=elevation10m.accu d8cut=500 memory=800 \


       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.


        r.flow, r.basins.fill, r.drain, r.topidx, r.topmodel, r.water.outlet, r.watershed


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

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

       Contact:  Laura Toma

       Last changed: $Date: 2017-02-11 22:22:16 +0100 (Sat, 11 Feb 2017) $


       Available at: r.terraflow source code (history)

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