## 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 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, Spherical 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