# dimfilter

Directional filtering of grids in the space domain

## Synopsis

**gmt dimfilter** *ingrid*
**-D***flag*
**-F****x***width*[*modifier*]
**-G***outgrid*
**-N****x***sectors*[*modifier*]
[ **-L** ]
[ **-Q** ]
[ **-I***increment* ]
[ **-R***region* ]
[ **-T** ]
[ **-V**[*level*] ]
[ **-f**flags ]
[ **-h**headers ]
[ **--PAR**=*value* ]

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

## Description

**dimfilter** will filter a *grid* file in the space (or time) domain by
dividing the given filter circle into *n_sectors*, applying one of the
selected primary convolution or non-convolution filters to each sector,
and choosing the final outcome according to the selected secondary
filter. It computes distances using Cartesian or Spherical geometries.
The output gridfile can optionally be generated as a subregion of the
input and/or with a new **-I**ncrement. In this way, one may have
“extra space” in the input data so that there will be no edge effects
for the output grid. If the filter is low-pass, then the output may be
less frequently sampled than the input. The **-Q** option is for the error analysis
mode and expects the input file to contains the filtered depths. Finally, one should know that
**dimfilter** will not produce a smooth output as other spatial filters
do because it returns a minimum median out of *N* medians of *N*
sectors. The output can be rough unless the input data are noise-free.
Thus, an additional filtering (e.g., Gaussian via grdfilter) of the
DiM-filtered data is generally recommended.

## Required Arguments

*ingrid*[=*ID*|?*varname*][**+b***band*][**+d***divisor*][**+n***invalid*][**+o***offset*][**+s***scale*]

Optionally, append =

IDfor reading a specific file format [Default is =nf] or ?varnamefor a specific netCDF variable [Default is the first 2-D grid found by GMT]. The following modifiers are supported:

+b- Select aband[Default is 0].

+d- Divide data values by the givendivisor[Default is 1].

+n- Replace data values matchinginvalidwith NaN.

+o- Offset data values by the givenoffset[Default is 0].

+s- Scale data values by the givenscale[Default is 1].

Note: Any offset is added after any scaling.

**-D***flag*Distance

*flag*(0-4) determines how grid (*x, y*) relates to filter*width*, as follows:*flag*= 0: grid (*x, y*) in 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 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.

**-F****x***width*[**+l**|**u**]Sets the primary filter type. Choose among convolution and non-convolution filters. Append the filter code

**x**followed by the full diameter*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**+l**or**+h**to the filter width if you rather want to return the smallest or largest of each sector’s modal values.

**-N****x***sectors*[**+l**|**u**]Sets the secondary filter type

**x**and the number of bow-tie sectors.*sectors*must be integer and larger than 0. When*sectors*is set to 1, the secondary filter is not effective. Available secondary filters**x**are:(

**l**) Lower: Return the minimum of all filtered values.(

**u**) Upper: Return the maximum of all filtered values.(

**a**) Average: Return the mean of all filtered values.(

**m**) Median: Return the median of all filtered values.(

**p**) Mode: Return the mode of all filtered values: If more than one mode is found we return their average value. Append**+l**or**+h**to the sectors if you rather want to return the smallest or largest of the modal values.

**-G***outgrid*[=*ID*][**+d***divisor*][**+n***invalid*][**+o***offset*|**a**][**+s***scale*|**a**][:*driver*[*dataType*][**+c***options*]]

Optionally, append =

IDfor writing a specific file format. The following modifiers are supported:

+d- Divide data values by givendivisor[Default is 1].

+n- Replace data values matchinginvalidwith a NaN.

+o- Offset data values by the givenoffset, or appendafor automatic range offset to preserve precision for integer grids [Default is 0].

+s- Scale data values by the givenscale, or appendafor automatic scaling to preserve precision for integer grids [Default is 1].

Note: Any offset is added before any scaling.+saalso sets+oa(unless overridden). To write specific formats via GDAL, use =gdand supplydriver(and optionallydataType) and/or one or more concatenated GDAL-cooptions using+c. See the “Writing grids and images” cookbook section for more details.

## Optional Arguments

**-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.]

**-L**This option is used by itself to write the

*dim.template.sh*bash script to standard output. No other options can be used in combination.

**-R***west*,*east*,*south*, and*north*defines the Region of the output grid. [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].

**-Q**For this mode, it expects to read depths consisted of several columns. Each column represents a filtered grid with a filter width, which can be obtained by

**grd2xyz -Z**. The outcome will be median, MAD, and mean. So, the column with the medians is used to generate the regional component and the column with the MADs is used to conduct the error analysis.

**-V**[*level*]Select verbosity level [

**w**]. (See full description) (See technical reference).

**-f**[**i**|**o**]*colinfo*(more …)Specify data types of input and/or output columns.

**-h**[**i**|**o**][*n*][**+c**][**+d**][**+m***segheader*][**+r***remark*][**+t***title*] (more …)Skip or produce header record(s).

**-^**or just**-**Print a short message about the syntax of the command, then exit (

**Note**: on Windows just use**-**).**-+**or just**+**Print an extensive usage (help) message, including the explanation of any module-specific option (but not the GMT common options), then exit.

**-?**or no argumentsPrint a complete usage (help) message, including the explanation of all options, then exit.

**--PAR**=*value*Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.

## 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

**Note**: Below are some examples of valid syntax for this module.
The examples that use remote files (file names starting with `@`

)
can be cut and pasted into your terminal for testing.
Other commands requiring input files are just dummy examples of the types
of uses that are common but cannot be run verbatim as written.

We have the remote file @earth_relief_05m which is a file of 5 minute bathymetry, 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. To prevent the medians from being biased by the sloping plane, you want to divide the filter circle into 6 sectors and to choose the lowest value among 6 medians. Using spherical distance calculations, you need:

```
gmt dimfilter @earth_relief_05m -Gfiltered_pacific.nc -Fm600 -D4 -Nl6 -R150/250/10/40 -I0.5 -V
```

Suppose that cape_verde.nc is a file of 0.5 minute bathymetry from 32W to 15W and 8N to 25N, and you want to remove small-length-scale features in order to define a swell in an area extending from 27.5W to 20.5W and 12.5N to 19.5N, and you want the output value every 2 minute. Using cartesian distance calculations, you need:

```
gmt dimfilter cape_verde.nc -Gt.nc -Fm220 -Nl8 -D2 -R-27.5/-20.5/12.5/19.5 -I2m -V
gmt grdfilter t.nc -Gcape_swell.nc -Fg50 -D2 -V
```

Suppose that you found a range of filter widths for a given area, and
you filtered the given bathymetric data using the range of filter widths
(e.g., *f100.nc f110.nc f120.nc f130.nc*), and you want to define a
regional trend using the range of filter widths, and you want to obtain
median absolute deviation (MAD) estimates at each data point. Then, you
will need to do:

```
gmt grd2xyz f100.nc -Z > f100.txt
gmt grd2xyz f110.nc -Z > f110.txt
gmt grd2xyz f120.nc -Z > f120.txt
gmt grd2xyz f130.nc -Z > f130.txt
paste f100.txt f110.txt f120.txt f130.txt > depths.txt
gmt dimfilter depths.txt -Q > output.z
```

## Limitations

When working with geographic (lat, lon) grids, all three convolution
filters (boxcar, cosine arch, and gaussian) will properly normalize the
filter weights for the variation in gridbox size with latitude, and
correctly determine which nodes are needed for the convolution when the
filter “circle” crosses a periodic (0-360) boundary or contains a
geographic pole. However, the spatial filters, such as median and mode
filters, do not use weights and thus should only be used on Cartesian
grids (or at very low latitudes) only. If you want to apply such spatial
filters you should project your data to an equal-area projection and run
**dimfilter** on the resulting Cartesian grid.

## Script Template

The dim.template.sh is a skeleton shell script that can be used to set
up a complete DiM analysis, including the MAD analysis. It is obtained
via the **-L** option.

## References

Kim, S.-S., and Wessel, P. (2008), Directional Median Filtering for
Regional-Residual Separation of Bathymetry, *Geochem. Geophys.
Geosyst.*, **9**, Q03005, https://doi.org/10.1029/2007GC001850.