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NAME

       r.fill.dir   -  Filters  and generates a depressionless elevation map and a flow direction
       map from a given elevation raster map.

KEYWORDS

       raster, hydrology, sink, fill sinks, depressions

SYNOPSIS

       r.fill.dir
       r.fill.dir --help
       r.fill.dir [-f]  input=name  output=name  direction=name   [areas=name]    [format=string]
       [--overwrite]  [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:
       -f
           Find unresolved areas only

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

       output=name [required]
           Name for output depressionless elevation raster map

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

       areas=name
           Name for output raster map of problem areas

       format=string
           Aspect direction format
           Options: agnps, answers, grass
           Default: grass

DESCRIPTION

       r.fill.dir  filters  and generates a depressionless elevation map and a flow direction map
       from a given raster elevation map.  The method adopted to filter  the  elevation  map  and
       rectify  it  is  based  on the paper titled "Extracting topographic structure from digital
       elevation model data for geographic information system analysis" by S.K. Jenson  and  J.O.
       Domingue (1988).

       The  procedure  takes  an elevation layer as input and initially fills all the depressions
       with one pass across the layer. Next, the flow direction algorithm tries to find a  unique
       direction  for  each  cell.  If  the  watershed  program  detects areas with pothholes, it
       delineates this area from the rest of the area and once again the depressions  are  filled
       using the neighborhood technique used by the flow direction routine. The final output will
       be a depressionless elevation layer and a unique flow direction layer.

       This (D8) flow algorithm performs as follows: At each raster cell the code determines  the
       slope  to  each  of  the 8 surrounding cells and assigns the flow direction to the highest
       slope out of the cell.  If there is more than one equal,  non-zero  slope  then  the  code
       picks  one  direction  based  on preferences that are hard-coded into the program.  If the
       highest slope is flat and in more than one direction then the code first tries  to  select
       an  alternative  based  on flow directions in the adjacent cells. r.fill.dir iterates that
       process, effectively propagating flow directions from areas where the directions are known
       into the area where the flow direction cannot otherwise be resolved.

       The  format  parameter  is  the type of format at which the user wishes to create the flow
       direction map.  The flow direction map can be encoded in GRASS GIS aspect format,  ANSWERS
       (Beasley et.al, 1982), or AGNPS (Young et.al, 1985) format, so that it can be readily used
       as input to other GRASS GIS modules or the aforementioned hydrological models.  The  grass
       format  gives  the same category values as r.slope.aspect gives for aspect, i.e. angles in
       degrees counter-clockwise from east in 45  degree  increments.   The  agnps  format  gives
       category  values  from  1-8,  with  1  facing north and increasing values in the clockwise
       direction.   The  answers  format  gives  category  values  from  0-360  degrees,  with  0
       (represented  as 360) facing east and values increasing in the counter-clockwise direction
       at 45 degree increments.  In all cases, NULL (no data) values are  used  for  cells  where
       direction cannot be determined.

       In  case  of local problems, those unfilled areas can be stored optionally.  Each unfilled
       area in this maps is numbered. The -f flag instructs the program to fill single-cell  pits
       but  otherwise to just find the undrained areas and exit. With the -f flag set the program
       writes an elevation map with just single-cell pits filled, a direction map with unresolved
       problems  and a map of the undrained areas that were found but not filled. This option was
       included because filling DEMs was often not the best way  to  solve  a  drainage  problem.
       These  options  let  the  user get a partially-fixed elevation map, identify the remaining
       problems and fix the problems appropriately.

       In some cases it may be necessary to run r.fill.dir repeatedly (using output from one  run
       as input to the next run) before all of problem areas are filled.

       The  resulting  depressionless  elevation  raster  map  can further be processed to derive
       slopes and other attributes required by other hydrological models.

       As any GRASS GIS module, r.fill.dir is sensitive to  the  computational  region  settings.
       Thus  the  module can be used to generate a flow direction map for any sub-area within the
       full map layer. Also, r.fill.dir is sensitive to any raster MASK in effect.

NOTES

           ·   The r.fill.dir module can be used not only to fill depression, but also to  detect
               water  bodies or potential water bodies based on the nature of the terrain and the
               digital elevation model used.

           ·   Not all depressions are errors in digital elevation  models.  In  fact,  many  are
               wetlands  and  as  Jenkins  and  McCauley  (2006)  note careless use of depression
               filling may lead to unintended consequences such as loss of wetlands.

           ·   Although  many  hydrological  algorithms  require  depression  filling,   advanced
               algorithms such as those implemented in r.watershed and r.sim.water do not require
               depressionless digital elevation model to work.

           ·   The flow direction map can be visualized with d.rast.arrow.

EXAMPLES

       Generic example: create a depressionless (sinkless) elevation  map  ansi.fill.elev  and  a
       flow direction map ansi.asp for the type "grass":
       r.fill.dir input=ansi.elev output=ansi.fill.elev direction=ansi.asp

       North  Carolina sample dataset example: The LiDAR derived 1m elevation map is sink-filled.
       The outcome are a depressionless elevation map, the flow direction map and an error map.
       # set computational region to elevation map
       g.region raster=elev_lid792_1m -p
       # generate depressionless DEM and related maps
       r.fill.dir input=elev_lid792_1m output=elev_lid792_1m_filled \
                  direction=elev_lid792_1m_dir areas=elev_lid792_1m_error
       # generate elevation map of pixelwise differences to see obtained terrain alterations
       r.mapcalc "elev_lid792_1m_diff = elev_lid792_1m_filled - elev_lid792_1m"
       r.colors elev_lid792_1m_diff color=differences
       # assess univariate statistics of differences
       r.univar -e elev_lid792_1m_diff
       # vectorize filled areas (here all fills are of positive value, see r.univar output)
       r.mapcalc "elev_lid792_1m_fill_area = if(elev_lid792_1m_diff > 0.0, 1, null() )"
       r.to.vect input=elev_lid792_1m_fill_area output=elev_lid792_1m_fill_area type=area
       # generate shaded terrain for better visibility of results
       r.relief input=elev_lid792_1m_filled output=elev_lid792_1m_filled_shade
       d.mon wx0
       d.shade shade=elev_lid792_1m_filled_shade color=elev_lid792_1m_filled
       d.vect elev_lid792_1m_fill_area type=boundary color=red
       Figure: Sink-filled DEM (shown as shaded terrain) with areas of filling  shown  as  vector
       polygons

REFERENCES

           ·   Beasley,  D.B.  and  L.F.  Huggins. 1982. ANSWERS (areal nonpoint source watershed
               environmental response simulation): User’s manual. U.S. EPA-905/9-82-001, Chicago,
               IL, 54 p.

           ·   Jenkins,  D.  G.,  and  McCauley,  L. A. 2006.  GIS, SINKS, FILL, and disappearing
               wetlands:  unintended  consequences  in  algorithm  development   and   use.    In
               Proceedings of the 2006 ACM symposium on applied computing (pp. 277-282).

           ·   Jenson,  S.K.,  and  J.O.  Domingue.  1988.  Extracting topographic structure from
               digital  elevation  model  data  for  geographic  information   system   analysis.
               Photogram.  Engr. and Remote Sens. 54: 1593-1600.

           ·   Young,  R.A.,  C.A.  Onstad,  D.D.  Bosch  and  W.P.  Anderson. 1985. Agricultural
               nonpoint surface pollution models (AGNPS) I and II model documentation. St.  Paul:
               Minn.  Pollution  control  Agency  and Washington D.C., USDA-Agricultural Research
               Service.

SEE ALSO

        d.rast.arrow, d.shade, g.region, r.fillnulls, r.relief, r.slope.aspect

AUTHORS

       Fortran  version:  Raghavan  Srinivasan,  Agricultural  Engineering   Department,   Purdue
       University
       Rewrite to C with enhancements: Roger S. Miller

       Last changed: $Date: 2017-11-24 02:59:27 +0100 (Fri, 24 Nov 2017) $

SOURCE CODE

       Available at: r.fill.dir source code (history)

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       © 2003-2019 GRASS Development Team, GRASS GIS 7.6.1 Reference Manual