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<title>GRDFILTER</title>
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<h1 align="center">GRDFILTER</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="#SEE ALSO">SEE ALSO</a><br>
<hr>
<h2>NAME
<a name="NAME"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">grdfilter
− Filter a 2-D grid file in the space (or time)
domain</p>
<h2>SYNOPSIS
<a name="SYNOPSIS"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>grdfilter</b>
<i>input_file</i> <b>−D</b><i>distance_flag</i>
<b>−F</b><i><filtertype><width></i>[<i>mode</i>]
<b>−G</b><i>output_file</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>−Ni</b>|<b>p</b>|<b>r</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> ] [
<b>−f</b>[<b>i</b>|<b>o</b>]<i>colinfo</i> ]</p>
<h2>DESCRIPTION
<a name="DESCRIPTION"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>grdfilter</b>
will filter a <i>.grd</i> file in the time domain using one
of the selected convolution or non-convolution isotropic
filters and compute distances using Cartesian or Spherical
geometries. The output <i>.grd</i> file can optionally be
generated as a subOPT(R)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.
<i><br>
input_file</i></p>
<p style="margin-left:22%;">The grid file of points to be
filtered. (See GRID FILE FORMATS below).</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><b>−D</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Distance <i>flag</i> tells how grid (x,y) relates to
filter <i>width</i> as follows:</p></td></tr>
</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. <i><br>
flag</i> = 5: grid (x,y) in Mercator -Jm1 img units,
<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"><b>−F</b></p></td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em">Sets the 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></tr>
</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, where <i>width</i> is 6 times the conventional
Gaussian sigma. <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. <br>
(<b>l</b>) Lower: Return the minimum of all values. <br>
(<b>L</b>) Lower: Return minimum of all positive values
only. <br>
(<b>u</b>) Upper: Return maximum of all values. <br>
(<b>U</b>) Upper: Return maximum or all negative values
only. <br>
In the case of <b>L</b>|<b>U</b> it is possible that no data
passes the initial sign test; in that case the filter will
return 0.0.</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"><b>−G</b></p></td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em"><i>output_file</i> is the output
grid file of the filter. (See GRID FILE FORMATS below).</p></td></tr>
</table>
<h2>OPTIONS
<a name="OPTIONS"></a>
</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"><b>−I</b></p></td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em"><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>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−N</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Determine how NaN-values in the input grid affects the
filtered outout: Append <b>i</b> to ignore all NaNs in the
calculation of filtered value [Default], <b>r</b> is same as
<b>i</b> except if the input node was NaN then the output
node will be set to NaN (only applies if both grids are
coregistered), and <b>p</b> which will force the filtered
value to be NaN if any grid-nodes with NaN-values are found
inside the filter circle.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−R</b></p></td>
<td width="8%"></td>
<td width="78%">
<p><i>west, east, south,</i> and <i>north</i> defines the
Region of the output points. [Default: Same as input.]</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−T</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>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>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−V</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Selects verbose mode, which will send progress reports
to stderr [Default runs "silently"].</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−f</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Special formatting of input and/or output columns (time
or geographical data). Specify <b>i</b> or <b>o</b> to make
this apply only to input or output [Default applies to
both]. Give one or more columns (or column ranges) separated
by commas. Append <b>T</b> (absolute calendar time),
<b>t</b> (relative time in chosen <b><A HREF="gmtdefaults.html#TIME_UNIT">TIME_UNIT</A></b> since
<b><A HREF="gmtdefaults.html#TIME_EPOCH">TIME_EPOCH</A></b>), <b>x</b> (longitude), <b>y</b>
(latitude), or <b>f</b> (floating point) to each column or
column range item. Shorthand
<b>−f</b>[<b>i</b>|<b>o</b>]<b>g</b> means
<b>−f</b>[<b>i</b>|<b>o</b>]0<b>x</b>,1<b>y</b>
(geographic coordinates).</p></td></tr>
</table>
<h2>GRID FILE FORMATS
<a name="GRID FILE FORMATS"></a>
</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>
<h2>GEOGRAPHICAL AND TIME COORDINATES
<a name="GEOGRAPHICAL AND TIME COORDINATES"></a>
</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>
<h2>EXAMPLES
<a name="EXAMPLES"></a>
</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. Using spherical distance calculations, you
need:</p>
<p style="margin-left:11%; margin-top: 1em"><b>grdfilter</b>
north_pacific_dbdb5.grd <b>−G</b> filtered_pacific.grd
<b>−Fm</b> 600 <b>−D</b> 4 <b>−R</b>
150/250/10/40 <b>−I</b> 0.5 <b>−V</b></p>
<h2>LIMITATIONS
<a name="LIMITATIONS"></a>
</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 grdfilter on the resulting Cartesian grid. <br>
To use the <b>−D</b> 5 option the input Mercator grid
must be created by img2mercgrd using the <b>−C</b>
option so the origin of the y-values is the Equator (i.e., x
= y = 0 correspond to lon = lat = 0).</p>
<h2>SEE ALSO
<a name="SEE ALSO"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><i><A HREF="GMT.html">GMT</A></i>(1),
<i><A HREF="grdfft.html">grdfft</A></i>(1) <i><A HREF="img2mercgrd.html">img2mercgrd</A></i>(1)</p>
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