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<title>DIMFILTER</title>
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<h1 align=center>DIMFILTER</h1>
<a href="#NAME">NAME</a><br>
<a href="#SYNOPSIS">SYNOPSIS</a><br>
<a href="#DESCRIPTION">DESCRIPTION</a><br>
<a href="#OPTIONS">OPTIONS</a><br>
<a href="#GRID FILE FORMATS">GRID FILE FORMATS</a><br>
<a href="#GEOGRAPHICAL AND TIME COORDINATES">GEOGRAPHICAL AND TIME COORDINATES</a><br>
<a href="#EXAMPLES">EXAMPLES</a><br>
<a href="#LIMITATIONS">LIMITATIONS</a><br>
<a href="#SCRIPT TEMPLATE">SCRIPT TEMPLATE</a><br>
<a href="#REFERENCE">REFERENCE</a><br>
<a href="#SEE ALSO">SEE ALSO</a><br>
<hr>
<a name="NAME"></a>
<h2>NAME</h2>
<p style="margin-left:11%; margin-top: 1em">dimfilter
− Directional filtering of 2-D gridded files in the
space (or time) domain</p>
<a name="SYNOPSIS"></a>
<h2>SYNOPSIS</h2>
<p style="margin-left:11%; margin-top: 1em"><b>dimfilter</b>
<i>input_file.grd</i> <b>−D</b><i>distance_flag</i>
<b>−F</b><i><filtertype><width></i>[<i>mode</i>]
<b>−G</b><i>output_file.grd</i>
<b>−N</b><i><filtertype><n_sectors></i>
<b>−Q</b><i>cols</i> [
<b>−I</b><i>xinc</i>[<i>unit</i>][<b>=</b>|<b>+</b>][/<i>yinc</i>[<i>unit</i>][<b>=</b>|<b>+</b>]]
] [
<b>−R</b><i>west</i>/<i>east</i>/<i>south</i>/<i>north</i>[<b>r</b>]
] [ <b>−T</b> ] [ <b>−V</b> ]</p>
<a name="DESCRIPTION"></a>
<h2>DESCRIPTION</h2>
<p style="margin-left:11%; margin-top: 1em"><b>dimfilter</b>
will filter a <i>.grd</i> file in the space (or time) domain
by dividing the given filter circle into <i>n_sectors</i>,
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 <i>.grd</i> file can optionally be generated as a
sub<b>−R</b>egion of the input and/or with a new
<b>−I</b>ncrement. 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.
<b>-Q</b> is for the error analysis mode and only requires
the total number of columns in the input file, which
contains the filtered depths. Finally, one should know that
<b>dimfilter</b> will not produce a smooth output as other
spatial filters do because it returns a minimum median out
of <i>N</i> medians of <i>N</i> sectors. The output can be
edgy unless the input data is noise-free. Thus, an
additional filtering (e.g., Gaussian) to the DiM-filtered
data is generally recommended. <i><br>
input_file.grd</i></p>
<p style="margin-left:22%;">The file of points to be
filtered.</p>
<table width="100%" border=0 rules="none" frame="void"
cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−D</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top">Distance
<i>flag</i> tells how grid (x,y) relates to filter
<i>width</i> as follows:</p></td>
</table>
<p style="margin-left:22%; margin-top: 1em"><i>flag</i> =
0: grid (x,y) same units as <i>width</i>, Cartesian
distances. <i><br>
flag</i> = 1: grid (x,y) in degrees, <i>width</i> in
kilometers, Cartesian distances. <i><br>
flag</i> = 2: grid (x,y) in degrees, <i>width</i> in km, dx
scaled by cos(middle y), Cartesian distances.</p>
<p style="margin-left:22%; margin-top: 1em">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.</p>
<p style="margin-left:22%; margin-top: 1em"><i>flag</i> =
3: grid (x,y) in degrees, <i>width</i> in km, dx scaled by
cosine(y), Cartesian distance calculation. <i><br>
flag</i> = 4: grid (x,y) in degrees, <i>width</i> in km,
Spherical distance calculation.</p>
<table width="100%" border=0 rules="none" frame="void"
cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−F</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top">Sets the primary
filter type. Choose among convolution and non-convolution
filters. Append the filter code followed by the full
diameter <i>width</i>. Available convolution filters
are:</p> </td>
</table>
<p style="margin-left:22%;">(<b>b</b>) Boxcar: All weights
are equal. <br>
(<b>c</b>) Cosine Arch: Weights follow a cosine arch curve.
<br>
(<b>g</b>) Gaussian: Weights are given by the Gaussian
function. <br>
Non-convolution filters are: <br>
(<b>m</b>) Median: Returns median value. <br>
(<b>p</b>) 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.</p>
<table width="100%" border=0 rules="none" frame="void"
cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−N</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top">Sets the secondary
filter type and the number of bow-tie sectors.
<i>n_sectors</i> must be integer and larger than 0. When
<i>n_sectors</i> is set to 1, the secondary filter is not
effective. Available secondary filters are:</p></td>
</table>
<p style="margin-left:22%;">(<b>l</b>) Lower: Return the
minimum of all filtered values. <br>
(<b>u</b>) Upper: Return the maximum of all filtered values.
<br>
(<b>a</b>) Average: Return the mean of all filtered values.
<br>
(<b>m</b>) Median: Return the median of all filtered values.
<br>
(<b>p</b>) Mode: Return the mode of all filtered values.</p>
<table width="100%" border=0 rules="none" frame="void"
cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−G</b></p> </td>
<td width="8%"></td>
<td width="67%">
<p style="margin-top: 1em" valign="top"><i>output_file.grd</i>
is the output of the filter.</p></td>
<td width="11%">
</td>
</table>
<a name="OPTIONS"></a>
<h2>OPTIONS</h2>
<table width="100%" border=0 rules="none" frame="void"
cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−I</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top"><i>x_inc</i> [and
optionally <i>y_inc</i>] is the output Increment. Append
<b>m</b> to indicate minutes, or <b>c</b> to indicate
seconds. If the new <i>x_inc, y_inc</i> are NOT integer
multiples of the old ones (in the input data), filtering
will be considerably slower. [Default: Same as input.]</p></td>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−R</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top"><i>west, east,
south,</i> and <i>north</i> defines the Region of the output
points. [Default: Same as input.]</p></td>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−T</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top">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].</p></td>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−Q</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top"><i>cols</i> is the
total number of columns in the input file. 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 to conduct the error analysis.</p></td>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−V</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top">Selects verbose
mode, which will send progress reports to stderr [Default
runs "silently"].</p></td>
</table>
<a name="GRID FILE FORMATS"></a>
<h2>GRID FILE FORMATS</h2>
<p style="margin-left:11%; margin-top: 1em">By default
<b><A HREF="GMT.html">GMT</A></b> writes out grid as single precision floats in a
COARDS-complaint netCDF file format. However, <b><A HREF="GMT.html">GMT</A></b> 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 2- or 4-byte integers. To specify the precision,
scale and offset, the user should add the suffix
<b>=</b><i>id</i>[<b>/</b><i>scale</i><b>/</b><i>offset</i>[<b>/</b><i>nan</i>]],
where <i>id</i> is a two-letter identifier of the grid type
and precision, and <i>scale</i> and <i>offset</i> are
optional scale factor and offset to be applied to all grid
values, and <i>nan</i> is the value used to indicate missing
data. When reading grids, the format is generally
automatically recognized. If not, the same suffix can be
added to input grid file names. See <b><A HREF="grdreformat.html">grdreformat</A></b>(1)
and Section 4.17 of the GMT Technical Reference and Cookbook
for more information.</p>
<p style="margin-left:11%; margin-top: 1em">When reading a
netCDF file that contains multiple grids, <b><A HREF="GMT.html">GMT</A></b> will
read, by default, the first 2-dimensional grid that can find
in that file. To coax <b><A HREF="GMT.html">GMT</A></b> into reading another
multi-dimensional variable in the grid file, append
<b>?</b><i>varname</i> to the file name, where
<i>varname</i> is the name of the variable. Note that you
may need to escape the special meaning of <b>?</b> 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 <b>?</b><i>varname</i> suffix can also be used
for output grids to specify a variable name different from
the default: "z". See <b><A HREF="grdreformat.html">grdreformat</A></b>(1) and
Section 4.18 of the GMT Technical Reference and Cookbook for
more information, particularly on how to read splices of 3-,
4-, or 5-dimensional grids.</p>
<a name="GEOGRAPHICAL AND TIME COORDINATES"></a>
<h2>GEOGRAPHICAL AND TIME COORDINATES</h2>
<p style="margin-left:11%; margin-top: 1em">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 <b>−f</b> or
<b>−R</b> options. For example, both <b>−f0x
−f1t</b> and <b>−R</b>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 <b><A HREF="gmtdefaults.html#TIME_UNIT">TIME_UNIT</A></b> and <b><A HREF="gmtdefaults.html#TIME_EPOCH">TIME_EPOCH</A></b>
in the .gmtdefaults file or on the command line. In
addition, the <b>unit</b> attribute of the time variable
will indicate both this unit and epoch.</p>
<a name="EXAMPLES"></a>
<h2>EXAMPLES</h2>
<p style="margin-left:11%; margin-top: 1em">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. 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:</p>
<p style="margin-left:11%; margin-top: 1em"><b>dimfilter</b>
north_pacific_dbdb5.grd <b>−G</b>filtered_pacific.grd
<b>−Fm</b>600 <b>−D</b>4 <b>−N</b>l6
<b>−R</b>150/250/10/40 <b>−I</b>0.5
<b>−V</b></p>
<p style="margin-left:11%; margin-top: 1em">Suppose that
cape_verde.grd 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:</p>
<p style="margin-left:11%; margin-top: 1em"><b>dimfilter</b>
cape_verde.grd <b>−G</b>t.grd <b>−Fm</b>220
<b>−Nl</b>8 <b>−D</b>2
<b>−R</b>-27.5/-20.5/12.5/19.5 <b>−I</b>2m
<b>−V <br>
grdfilter</b> t.grd <b>−G</b>cape_swell.grd
<b>−Fg</b>50 <b>−D</b>2 <b>−V</b></p>
<p style="margin-left:11%; margin-top: 1em">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., <i>f100.grd f110.grd f120.grd
f130.grd</i>), 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, you
need:</p>
<p style="margin-left:11%; margin-top: 1em"><b><A HREF="grd2xyz.html">grd2xyz</A></b>
f100.grd <b>-Z</b> > f100.d <b><br>
grd2xyz</b> f110.grd <b>-Z</b> > f110.d <b><br>
grd2xyz</b> f120.grd <b>-Z</b> > f120.d <b><br>
grd2xyz</b> f130.grd <b>-Z</b> > f130.d <b><br>
paste</b> f100.d f110.d f120.d f130.d > depths.d <b><br>
dimfilter</b> depths.d <b>-Q</b>4 > output.z</p>
<a name="LIMITATIONS"></a>
<h2>LIMITATIONS</h2>
<p style="margin-left:11%; margin-top: 1em">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.</p>
<a name="SCRIPT TEMPLATE"></a>
<h2>SCRIPT TEMPLATE</h2>
<p style="margin-left:11%; margin-top: 1em">The
dim.template.sh is a skeleton shell script that can be used
to set up a complete DiM analysis, including the MAD
analysis.</p>
<a name="REFERENCE"></a>
<h2>REFERENCE</h2>
<p style="margin-left:11%; margin-top: 1em">Kim, S.-S., and
Wessel, P. (2008), Directional Median Filtering for
Regional-Residual Separation of Bathymetry, <i>Geochem.
Geophys. Geosyst., 9(Q03005)</i>,
doi:10.1029/2007GC001850.</p>
<a name="SEE ALSO"></a>
<h2>SEE ALSO</h2>
<p style="margin-left:11%; margin-top: 1em"><i><A HREF="GMT.html">GMT</A></i>(1),
<i><A HREF="grdfilter.html">grdfilter</A></i>(1)</p>
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