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


       raster, hydrology


       r.fill.dir help
       r.fill.dir   [-f]    input=name    elevation=string    direction=string     [areas=string]
       [type=string]   [--overwrite]  [--verbose]  [--quiet]

           Find unresolved areas only

           Allow output files to overwrite existing files

           Verbose module output

           Quiet module output

           Name of existing raster map containing elevation surface

           Output elevation raster map after filling

           Output direction raster map

           Output raster map of problem areas

           Output aspect direction format (agnps, answers, or grass)
           Default: grass


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


       The type parameter is the type of format at which the  user  wishes  to  create  the  flow
       direction  map.  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  (360)  facing  east  and  values increasing in the counter
       clockwise direction at 45 degree increments. The grass  format  gives  the  same  category
       values as the r.slope.aspect program.

       The method adopted to filter the elevation map and rectify it is based on the paper titled
       "Software  Tools  to  Extract  Structure  from  Digital  Elevation  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 iteratates that
       process, effectively propagating flow directions from areas where the directions are known
       into the area where the flow direction can't otherwise be resolved.

       The  flow  direction  map  can be encoded in either ANSWERS (Beasley, 1982) or AGNPS
       (Young, 1985) form, so that it can be readily used  as  input  to  these  hydrologic
       models.  The  resulting  depressionless  elevation  layer  can  further be manipulated for
       deriving slopes and other attributes required by the hydrologic models.

       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.

       r.fill.dir  is  sensitive  to  the current window setting. Thus the program 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 mask in effect.

       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.


       r.fill.dir input=ansi.elev elevation=ansi.fill.elev direction=ansi.asp

       will create a depressionless (sinkless) elevation map ansi.fill.elev and a flow  direction
       map ansi.asp for the type "grass".


        r.fillnulls, r.slope.aspect

       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

       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.


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

       Last changed: $Date: 2008-05-15 11:59:22 -0700 (Thu, 15 May 2008) $

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