grdfilter - Filter a .grd file in the Time domain


      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


      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 sampled than the input.

           The file of points to be filtered.

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

           flag = 0:  grid (x,y) same units as width, Cartesian distances.
           flag = 1:  grid (x,y) in degrees, width in kilometers, Cartesian
           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, Spherical distance

      -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.


      -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

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

      -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"].


      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


      gmt, grdfft

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