NAME

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

KEYWORDS

       raster, elevation, interpolation

SYNOPSIS

       r.fillnulls
       r.fillnulls help
       r.fillnulls  input=string  output=string   [tension=float]   [smooth=float]  method=string
       [--overwrite]  [--verbose]  [--quiet]

   Flags:
       --overwrite
           Allow output files to overwrite existing files

       --verbose
           Verbose module output

       --quiet
           Quiet module output

   Parameters:
       input=string
           Name of input raster map in which to fill nulls

       output=string
           Name for output raster map with nulls filled by interpolation

       tension=float
           Spline tension parameter
           Default: 40.

       smooth=float
           Spline smoothing parameter
           Default: 0.1

       method=string
           Interpolation method
           Options: bilinear,bicubic,rst
           Default: rst

DESCRIPTION

       r.fillnulls fills NULL pixels (no data areas) in input map and stores filled map to a  new
       output  map.  The fill data are interpolated from the no data area boundaries buffer using
       v.surf.rst or v.surf.bspline interpolation.

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.

       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.

NOTES

       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 bilinear or bicubic method can be selected (based on v.surf.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  d.what.rast  to query
       individual cell values.

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 rast=elev_srtm_30m -p
       d.mon x0
       d.histogram elev_srtm_30m
       # remove too low elevations (esp. lakes)
       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
       r.fillnulls input=elev_srtm_30m_filt output=elev_srtm_30m_complete tension=20
       d.histogram elev_srtm_30m_complete
       d.rast elev_srtm_30m_complete

SEE ALSO

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

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.

AUTHORS

       r.fillnulls: Markus Neteler, University of Hannover and Fondazione Edmund Mach

       and authors of v.surf.rst
       Improvement by Hamish Bowman, NZ

       Last changed: $Date: 2012-03-04 03:55:32 -0800 (Sun, 04 Mar 2012) $

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