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       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]] ] [ -Rwest/east/south/north[r] ] [ -T ] [ -V ]


       grdfilter will filter a .grd file  in  the  time  domain  using  a  boxcar,  cosine  arch,
       gaussian,  median,  or  mode  filter  and computing distances using Cartesian or Spherical
       geometries.  The output .grd file can optionally be generated as a sub-Region of the input
       and/or with 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.  If the filter is low-pass, then the output may be less frequently sampled than the

              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

              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.


       -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.]

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

       -T     Toggle  the  node  registration for the output grid so as to become the opposite of
              the input grid [Default gives the same registration as the input grid].

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


       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


       gmt(1gmt), grdfft(1gmt)

                                            1 Jan 2004                               GRDFILTER(l)