NAME

     grdfilter - Filter a .grd file in the Time domain

SYNOPSIS

     grdfilter  input_file.grd   -  Dflag   -  F<type><width>   -
     Goutput_file.grd  [  - Ix_inc[m|c][/y_inc[m|c]] ] [ -N ] [ -
     Rwest/east/south/north[r] ] [ -V ]

DESCRIPTION

     grdfilter will filter a .grd file in the time domain,  using
     a boxcar, cosine arch, gaussian, median, or mode filter, and
     computing distances by Cartesian  or  Spherical  geometries.
     The  output  .grd file can optionally be generated in a sub-
     Region of the input, and / or at a new -Increment.  In  this
     way,  one  may  have "extra space" in the input data so that
     the edges will not be used, and the  output  can  be  within
     one-half-width  of  the  input edges; also, if the filter is
     low-pass, then the output may be needed less frequently sam-
     pled than the input.

     input_file.grd
          The file of points to be filtered.

     -D    Distance flag tells how grid (x,y) relates  to  filter
          width as follows:

          flag = 0:  grid (x,y) same units  as  width,  Cartesian
          distance calculation.

          flag = 1:  grid (x,y) in degrees, width in  kilometers,
          Cartesian distance calculation.

          flag = 2:  grid (x,y)  in  degrees,  width  in  km,  dx
          scaled by cosine(middle y), Cartesian distance calcula-
          tion.

          The above options are fastest because they allow weight
          matrix  to be computed only once.  The next two options
          are slower because  they  recompute  weights  for  each
          East-West scan line.

          flag = 3:  grid (x,y)  in  degrees,  width  in  km,  dx
          scaled by cosine(y), Cartesian distance calculation.

          flag = 4:  grid (x,y) in degrees, width in km,  Spheri-
          cal distance calculation.

     -F    Choose one only of bcgmp for (b)oxcar, (c)osine  Arch,
          (g)aussian,    (m)edian,    or    maximum    likelihood
          (p)robability (a mode  estimator)  filter  and  specify
          full width.

     -G    output_file.grd is the output of the filter.

OPTIONS

     -I    x_inc [and optionally y_inc] is the output  Increment.
          Append m to indicate minutes, or c to indicate seconds.
          If  the new x_inc, y_inc are NOT integer  multiples  of
          the  old  ones  (in  the input data), filtering will be
          considerably slower.  [Default:  Same as input.]

     -N    Force output grid  to  be  pixel  registered  [Default
          gives gridline registration].

     -R    west, east, south, and north defines the Region of the
          output points.  [Default:  Same as input.]

     -V    Selects verbose mode, which will send progress reports
          to stderr [Default runs "silently"].

EXAMPLES

     Suppose that north_pacific_dbdb5.grd is a file of  5  minute
     bathymetry  from 140E to 260E and 0N to 50N, and you want to
     find the medians of values within a 300km radius (600km full
     width)  of  the  output  points, which you choose to be from
     150E to 250E and 10N to 40N, and you want the output  values
     every  0.5  degree.   Using spherical distance calculations,
     you need:

     grdfilter north_pacific_dbdb5.grd - Gfiltered_pacific.grd  -
     Fm600 -D4 -R150/250/10/40 -I0.5 -V

SEE ALSO

     gmt, grdfft

REFERENCES

     Wessel, P., and W. H. F. Smith, 1995,  The  Generic  Mapping
     Tools  (GMT)  version  3.0  Technical  Reference & Cookbook,
     SOEST/NOAA.
     Wessel, P., and W. H. F. Smith, 1995,  New  Version  of  the
     Generic Mapping Tools Released, EOS Trans. AGU, 76, p. 329.
     Wessel, P., and W. H. F. Smith, 1995,  New  Version  of  the
     Generic           Mapping           Tools          Released,
     http://www.agu.org/eos_elec/95154e.html, Copyright  1995  by
     the American Geophysical Union.
     Wessel, P., and W. H. F. Smith, 1991,  Free  Software  Helps
     Map and Display Data, EOS Trans. AGU, 72, p. 441.