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NAME

       r.fillnulls  - Fills no-data areas in raster maps using spline interpolation.

KEYWORDS

       raster, surface, elevation, interpolation, splines, no-data filling

SYNOPSIS

       r.fillnulls
       r.fillnulls --help
       r.fillnulls   input=name   output=name   method=string   [tension=float]    [smooth=float]
       [edge=integer]   [npmin=integer]    [segmax=integer]    [lambda=float]    [memory=integer]
       [--overwrite]  [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:
       --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 raster map

       output=name [required]
           Name for output raster map

       method=string [required]
           Interpolation method to use
           Options: bilinear, bicubic, rst
           Default: rst

       tension=float
           Spline tension parameter
           Default: 40.

       smooth=float
           Spline smoothing parameter
           Default: 0.1

       edge=integer
           Width of hole edge used for interpolation (in cells)
           Options: 2-100
           Default: 3

       npmin=integer
           Minimum number of points for approximation in a segment (>segmax)
           Options: 2-10000
           Default: 600

       segmax=integer
           Maximum number of points in a segment
           Options: 2-10000
           Default: 300

       lambda=float
           Tykhonov regularization parameter (affects smoothing)
           Used in bilinear and bicubic spline interpolation
           Default: 0.01

       memory=integer
           Maximum memory to be used (in MB)
           Cache size for raster rows
           Default: 300

DESCRIPTION

       r.fillnulls  fills  NULL pixels (no data areas) in input raster map and stores filled data
       to a new output raster map. The  fill  areas  are  interpolated  from  the  no  data  area
       boundaries   buffer   using  v.surf.rst  regularized  spline  interpolation  with  tension
       (method=rst) or r.resamp.bspline  cubic  or  linear  spline  interpolation  with  Tykhonov
       regularization.

NOTES

       Each  area boundary buffer is set to three times the map resolution to get nominally three
       points around the edge. This way the algorithm interpolates into the hole with  a  trained
       slope and curvature at the edges, in order to avoid that such a flat plane is generated in
       a hole. The width of edge area can be adjusted by changing the edge parameter.

       During the interpolation following warning may occur when using the RST method:

       Warning: strip exists with insufficient data
       Warning: taking too long to find points for interpolation--please  change  the  region  to
       area where your points are

       This  warning  is  generated if large data holes exist within the surface.  As the idea of
       r.fillnulls is to fill such holes, the user may ignore the warning. The interpolation will
       be continued. However, the user may pay attention to below notes.

       If  interpolation  fails,  temporary  raster  and  vector  maps are left in place to allow
       unfilled map hole (NULL area) identification and manual repair.

       When using the default RST method,  the  algorithm  is  based  on  v.surf.rst  regularized
       splines  with  tension  interpolation module which interpolates the raster cell values for
       NULL data areas from the boundary values of the NULL data area. An eventual raster MASK is
       respected  during  the NULL data area(s) filling. The interpolated values are patched into
       the NULL data area(s) of the input map and saved into a new raster map.  Otherwise, either
       the  linear  or  cubic  spline  interpolation with Tykhonov regularization can be selected
       (based on r.resamp.bspline).

WARNING

       Depending on the shape of the NULL data area(s) problems may occur due to an  insufficient
       number  of  input cell values for the interpolation process. Most problems will occur if a
       NULL data area reaches a large amount of the map boundary. The user will have to carefully
       check  the  result  using  r.mapcalc  (generating  a  difference  map to the input map and
       applying the "differences" color table with r.colors)  and/or  to  query  individual  cell
       values.

       RST method stores temporary maps on hard disk. It will require at least as much free space
       as one extra input raster map takes.

EXAMPLE

       In this example, the SRTM elevation map in the North Carolina sample dataset  location  is
       filtered for outlier elevation values; missing pixels are then re-interpolated to obtain a
       complete elevation map:
       g.region raster=elev_srtm_30m -p
       d.mon wx0
       d.histogram elev_srtm_30m
       # remove SRTM outliers, i.e. SRTM below 50m (esp. lakes), leading to no data areas
       r.mapcalc "elev_srtm_30m_filt = if(elev_srtm_30m < 50.0, null(), elev_srtm_30m)"
       d.histogram elev_srtm_30m_filt
       d.rast elev_srtm_30m_filt
       # using the default RST method to fill these holes in DEM
       r.fillnulls input=elev_srtm_30m_filt output=elev_srtm_30m_rst tension=20
       # using the bilinear method to fill these holes in DEM
       r.fillnulls input=elev_srtm_30m_filt output=elev_srtm_30m_bilin method=bilinear
       d.histogram elev_srtm_30m_rst
       d.rast elev_srtm_30m_rst
       d.erase
       d.histogram elev_srtm_30m_bilin
       d.rast elev_srtm_30m_bilin
       r.mapcalc "diff_rst_bilin = elev_srtm_30m_rst - elev_srtm_30m_bilin"
       r.colors diff_rst_bilin color=differences
       r.univar -e diff_rst_bilin
       d.erase
       d.rast diff_rst_bilin
       d.legend diff_rst_bilin

REFERENCES

           ·   Mitas,  L.,  Mitasova,  H.,  1999,  Spatial  Interpolation.  In:  P.Longley,  M.F.
               Goodchild,  D.J.  Maguire,  D.W.Rhind  (Eds.),  Geographical  Information Systems:
               Principles, Techniques, Management and Applications, Wiley, pp.481-492

           ·   Mitasova H., Mitas L.,  Brown W.M.,  D.P. Gerdes, I.  Kosinovsky,  Baker,  T.1995,
               Modeling spatially and temporally distributed phenomena: New methods and tools for
               GRASS GIS. International Journal of GIS, 9 (4), special issue on  Integrating  GIS
               and Environmental modeling, 433-446.

           ·   Mitasova  H.  and Mitas L. 1993: Interpolation by Regularized Spline with Tension:
               I.  Theory and Implementation, Mathematical Geology 25, 641-655.

           ·   Mitasova H.  and Hofierka  L.  1993:  Interpolation  by  Regularized  Spline  with
               Tension:  II.  Application  to  Terrain  Modeling  and  Surface Geometry Analysis,
               Mathematical Geology 25, 657-667.

SEE ALSO

        r.fill.dir, r.mapcalc, r.resamp.bspline, v.surf.rst

AUTHORS

       Markus Neteler, University of Hannover  and Fondazione Edmund Mach
       Improvement by Hamish Bowman, NZ

       Last changed: $Date: 2018-08-27 18:32:33 +0200 (Mon, 27 Aug 2018) $

SOURCE CODE

       Available at: r.fillnulls source code (history)

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