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  fre-
     quently sampled 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
          distances.
          flag = 1:  grid (x,y) in degrees, width in  kilometers,
          Cartesian distances.
          flag = 2:  grid (x,y)  in  degrees,  width  in  km,  dx
          scaled by cos(middle y), Cartesian distances.

          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(l), grdfft(l)