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)