grdfilter - Filter a .grd file in the Time domain
grdfilter input_file.grd -Dflag -F<type><width>[mode] -Goutput_file.grd
[ -Ix_inc[m|c][/y_inc[m|c]] ] [ -Rwest/east/south/north[r] ] [ -T ] [
grdfilter will filter a .grd file in the time domain using one of the
selected convolution or non-convolution filters and compute 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.
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 Sets the filter type. Choose among convolution and non-convolu-
tion filters. Append the filter code followed by the full diame-
ter width. Available convolution filters are:
(b) Boxcar: All weights are equal.
(c) Cosine Arch: Weights follow a cosine arch curve.
(g) Gaussian: Weights are given by the Gaussian function.
Non-convolution filters are:
(m) Median: Returns median value.
(p) Maximum likelihood probability (a mode estimator): Return
modal value. If more than one mode is found we return their
average value. Append - or + to the filter width if you rather
want to return the smallest or largest of the modal values.
(l) Lower: Return the minimum of all values.
(L) Lower: Return minimum of all positive values only.
(u) Upper: Return maximum of all values.
(U) Upper: Return maximum or all negative values only.
In the case of L|U it is possible that no data passes the ini-
tial sign test; in that case the filter will return 0.0.
-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 registra-
tion as the input grid].
-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 calcula-
tions, you need:
grdfilter north_pacific_dbdb5.grd -Gfiltered_pacific.grd -Fm600 -D4
-R150/250/10/40 -I0.5 -V
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 program for
the next major release.
GMT4.0 1 Oct 2004 GRDFILTER(l)
Man(1) output converted with