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grdfilter - Filter a grid in the space (or time) domain


grdfilter ingrid distance_flag xwidth[/width2][modifiers] outgrid [ increment ] [ i|p|r ]
[ region ] [ ] [ [level] ] [ -f<flags> ]

Note: No space is allowed between the option flag and the associated arguments.


grdfilter will filter a grid file in the time domain using one of the selected convolution
or non-convolution isotropic or rectangular filters 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 "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.


ingrid The grid file of points to be filtered. (See GRID FILE FORMATS below).

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

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

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

Sets the filter type. Choose among convolution and non-convolution 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).
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 initial sign test; in
that case the filter will return NaN.

outgrid is the output grid file of the filter. (See GRID FILE FORMATS below).


x_inc [and optionally y_inc] is the output Increment. Append m to indicate arc
minutes, or s to indicate arc 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.]

Determine how NaN-values in the input grid affects the filtered output: Append i to
ignore all NaNs in the calculation of filtered 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 filtered 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 registration as the input grid].

-V[level] (more ...)
Select verbosity level [c].

-f[i|o]colinfo (more ...)
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
use just -).

-+ or just +
Print an extensive usage (help) message, including the explanation 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 options, then

Print GMT version and exit.

Print full path to GMT share directory and exit.


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 "packing" of grids, writing out floating point
data as 1- or 2-byte integers. To specify the precision, scale and offset, the user should
add the suffix =id[/scale/offset[/nan]], where id is a two-letter identifier of the grid
type and precision, and scale and offset are optional scale factor and offset to be
applied to all grid values, and nan is the value used to indicate missing data. In case
the two characters id is not provided, as in =/scale than a id=nf is assumed. When
reading grids, the format is generally automatically recognized. If not, the same suffix
can be added to input grid file names. See grdconvert and Section grid-file-format of the
GMT Technical Reference and Cookbook for more information.

When reading a netCDF file that contains multiple grids, GMT will read, by default, the
first 2-dimensional grid that can find in that file. To coax GMT into reading another
multi-dimensional variable in the grid file, append ?varname to the file name, where
varname is the name of the variable. Note that you may need to escape the special meaning
of ? in your shell program by putting a backslash in front of it, or by placing the
filename and suffix between quotes or double quotes. The ?varname suffix can also be used
for output grids to specify a variable name different from the default: "z". See
grdconvert and Sections modifiers-for-CF and grid-file-format of the GMT Technical
Reference and Cookbook for more information, particularly on how to read splices of 3-,
4-, or 5-dimensional grids.


When the output grid type is netCDF, the coordinates will be labeled "longitude",
"latitude", or "time" 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.


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 calculations,
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 corresponding 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 -Fm-600 \
-D4 -R150/250/10/40 -I0.5 -V

Here, the residual_pacific.nc grid will have the same 5 minute resolution 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


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).

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