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]] ] [ -Rwest/east/south/north[r] ] [ -T ] [  -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
       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 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 dis-
              tance calculation.

       -F     Choose one only of bcgmp for (b)oxcar,  (c)osine  Arch,  (g)aus-
              sian,  (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.]

       -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 registra-
              tion as the input grid].

       -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  calcula-
       tions, you need:

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


BUGS

       grdfilter is not yet aware of boundary  conditions  and  periodicities;
       hence  filtering  global grids will not necessarily give you the result
       you seek.  You can remedy this partly by  padding  the  east  and  west
       sides  (e.g., use a grid that goes from -10 to 370 in longitude) but no
       such solution is possibly across the poles untill we rewrite  the  pro-
       gram for the next major release.


SEE ALSO

       gmt(l), grdfft(l)



VERSION                              DATE                         GRDFILTER(l)

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