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**PROGRAM:**

**NAME**

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

**SYNOPSIS**

**grdfilter**

__ingrid__

__distance_flag__

**x**

__width__[/

__width2__][

__modifiers__]

__outgrid__[

__increment__] [

**i**|

**p**|

**r**]

[

__region__] [ ] [ [

__level__] ] [

**-f**<flags> ]

**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 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.

**REQUIRED** **ARGUMENTS**

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

**-D**

__distance_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

**-Jm**1 img units,

__width__in km, Spherical distance

calculation.

**-Fx**

__width__

**[/**

__width2__

**][**

__modifiers__

**]**

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

**+q**

__quantile__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.

**-G**

__outgrid__

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

**OPTIONAL** **ARGUMENTS**

**-I**

__xinc__

**[**

__unit__

**][=|+][/**

__yinc__

**[**

__unit__

**][=|+]]**

__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.]

**-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 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

exits.

**--version**

Print GMT version and exit.

**--show-datadir**

Print full path to GMT share directory and exit.

**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 "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.

**GEOGRAPHICAL** **AND** **TIME** **COORDINATES**

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

**-R**90w/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 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

**LIMITATIONS**

To use the

**-D**5 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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