grdfilter(1) GMT grdfilter(1)
NAME
grdfilter - Filter a grid in the space (or time) domain
SYNOPSIS
grdfilter ingrid -Ddistance_flag
-Fxwidth[/width2][modifiers]
-Goutgrid [ -Iincrement ] [ -Ni|p|r ] [ -Rregion ] [ -T ] [
-V[level] ] [ -fflags ]
Note: No space is allowed between the option flag and the associated
arguments.
DESCRIPTION
grdfilter will filter a grid file in the time domain using one of the
selected convolution or non-convolution isotropic or rectangular fil-
ters and compute distances using Cartesian or Spherical geometries.
The output grid file can optionally be generated as a sub-region of the
input (via -R) and/or with new increment (via -I) or registration (via
-T). In this way, one may have aextra spacea 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.
REQUIRED ARGUMENTS
ingrid The grid file of points to be filtered. (See GRID FILE FORMATS
below).
-Ddistance_flag
Distance flag tells how grid (x,y) relates to filter width as
follows:
flag = p: grid (px,py) with width an odd number of pixels;
Cartesian distances.
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 three options are slower
because they recompute weights for each latitude.
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.
flag = 5: grid (x,y) in Mercator -Jm1 img units, width in km,
Spherical distance calculation.
-Fxwidth[/width2][modifiers]
Sets the filter type. Choose among convolution and non-convolu-
tion filters. Use any filter code x (listed below) followed by
the full diameter width. This gives an isotropic filter; append
/width2 for a rectangular filter (requires -Dp or -D0). By
default we perform low-pass filtering; append +h to select
high-pass filtering. Some filters allow for optional arguments
and modifiers.
Convolution filters (and their codes) 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, where
width is 6 times the conventional Gaussian sigma.
(f) Custom: Weights are given by the precomputed values in the
filter weight grid file weight, which must have odd dimensions;
also requires -D0 and output spacing must match input spacing or
be integer multiples.
(o) Operator: Weights are given by the precomputed values in the
filter weight grid file weight, which must have odd dimensions;
also requires -D0 and output spacing must match input spacing or
be integer multiples. Weights are assumed to sum to zero so no
accumulation of weight sums and normalization will be done.
Non-convolution filters (and their codes) are:
(m) Median: Returns median value. To select another quantile
append +qquantile in the 0-1 range [Default is 0.5, i.e.,
median].
(p) Maximum likelihood probability (a mode estimator): Return
modal value. If more than one mode is found we return their
average value. Append +l or +u if you rather want to return the
lowermost or uppermost of the modal values.
(h) Histogram mode (another mode estimator): Return the modal
value as the center of the dominant peak in a histogram. Append
/binwidth to specify the binning interval. Use modifier +c to
center the bins on multiples of binwidth [Default has bin edges
that are multiples of binwidth]. If more than one mode is found
we return their average value. Append +l or +u if you rather
want to return the lowermost or uppermost 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 NaN.
-Goutgrid
outgrid is the output grid file of the filter. (See GRID FILE
FORMATS below).
OPTIONAL ARGUMENTS
-Ixinc[unit][+e|n][/yinc[unit][+e|n]]
x_inc [and optionally y_inc] is the grid spacing. Optionally,
append a suffix modifier. Geographical (degrees) coordinates:
Append m to indicate arc minutes or s to indicate arc seconds.
If one of the units e, f, k, M, n or u is appended instead, the
increment is assumed to be given in meter, foot, km, Mile, nau-
tical mile or US survey foot, respectively, and will be con-
verted to the equivalent degrees longitude at the middle lati-
tude of the region (the conversion depends on PROJ_ELLIPSOID).
If y_inc is given but set to 0 it will be reset equal to x_inc;
otherwise it will be converted to degrees latitude. All coordi-
nates: If +e is appended then the corresponding max x (east) or
y (north) may be slightly adjusted to fit exactly the given
increment [by default the increment may be adjusted slightly to
fit the given domain]. Finally, instead of giving an increment
you may specify the number of nodes desired by appending +n to
the supplied integer argument; the increment is then recalcu-
lated from the number of nodes and the domain. The resulting
increment value depends on whether you have selected a grid-
line-registered or pixel-registered grid; see App-file-formats
for details. Note: if -Rgrdfile is used then the grid spacing
has already been initialized; use -I to override the values.
-Ni|p|r
Determine how NaN-values in the input grid affects the filtered
output: Append i to ignore all NaNs in the calculation of fil-
tered value [Default], r is same as i except if the input node
was NaN then the output node will be set to NaN (only applies if
both grids are co-registered), and p which will force the fil-
tered value to be NaN if any grid-nodes with NaN-values are
found inside the filter circle.
-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[level] (more a|)
Select verbosity level [c].
-f[i|o]colinfo (more a|)
Specify data types of input and/or output columns.
-^ or just -
Print a short message about the syntax of the command, then
exits (NOTE: on Windows just use -).
-+ or just +
Print an extensive usage (help) message, including the explana-
tion of any module-specific option (but not the GMT common
options), then exits.
-? or no arguments
Print a complete usage (help) message, including the explanation
of all options, then exits.
GRID FILE FORMATS
By default GMT writes out grid as single precision floats in a
COARDS-complaint netCDF file format. However, GMT is able to produce
grid files in many other commonly used grid file formats and also
facilitates so called apackinga of grids, writing out floating point
data as 1- or 2-byte integers. (more a|)
GEOGRAPHICAL AND TIME COORDINATES
When the output grid type is netCDF, the coordinates will be labeled
alongitudea, alatitudea, or atimea based on the attributes of the input
data or grid (if any) or on the -f or -R options. For example, both
-f0x -f1t and -R90w/90e/0t/3t will result in a longitude/time grid.
When the x, y, or z coordinate is time, it will be stored in the grid
as relative time since epoch as specified by TIME_UNIT and TIME_EPOCH
in the gmt.conf file or on the command line. In addition, the unit
attribute of the time variable will indicate both this unit and epoch.
EXAMPLES
Suppose that north_pacific_etopo5.nc 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:
gmt grdfilter north_pacific_etopo5.nc -Gfiltered_pacific.nc -Fm600 \
-D4 -R150/250/10/40 -I0.5 -V
If we instead wanted a high-pass result then one can perform the corre-
sponding low-pass filter using a coarse grid interval as grdfilter will
resample the result to the same resolution as the input grid so we can
compute the residuals, e.g.,
gmt grdfilter north_pacific_etopo5.nc -Gresidual_pacific.nc -Fm600+h \
-D4 -R150/250/10/40 -I0.5 -V
Here, the residual_pacific.nc grid will have the same 5 minute resolu-
tion as the original.
To filter the dataset in ripples.nc using a custom anisotropic Gaussian
filter exp (-0.5*r^2) whose distances r from the center is given by
(2x^2 + y^2 -2xy)/6, with major axis at an angle of 63 degrees with the
horizontal, try
gmt grdmath -R-10/10/-10/10 -I1 X 2 POW 2 MUL Y 2 POW ADD X Y MUL 2 MUL \
SUB 6 DIV NEG 2 DIV EXP DUP SUM DIV = gfilter.nc
gmt grdfilter ripples.nc -Ffgfilter.nc -D0 -Gsmooth.nc -V
LIMITATIONS
1. To use the -D5 option the input Mercator grid must be created by
img2mercgrd using the -C option so the origin of the y-values is the
Equator (i.e., x = y = 0 correspond to lon = lat = 0).
2. If the new x_inc, y_inc set with -I are NOT integer multiples of the
increments in the input data, filtering will be considerably slower.
[Default increments: Same as input.]
SEE ALSO
gmt(1), grdfft(1), img2grd(1)
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 grdfilter(1)
gmt5 5.4.2 - Generated Wed Jun 28 18:26:31 CDT 2017