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<title>GRDTREND</title>
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<h1 align=center>GRDTREND</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="#REMARKS">REMARKS</a><br>
<a href="#GRID FILE FORMATS">GRID FILE FORMATS</a><br>
<a href="#EXAMPLES">EXAMPLES</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">grdtrend
− Fit and/or remove a polynomial trend in a grid
file</p>
<a name="SYNOPSIS"></a>
<h2>SYNOPSIS</h2>
<p style="margin-left:11%; margin-top: 1em"><b>grdtrend</b>
<i>grdfile</i> <b>−N</b><i>n_model</i>[<b>r</b>] [
<b>−D</b><i>diff.grd</i> ] [
<b>−T</b><i>trend.grd</i> ] [ <b>−V</b> ] [
<b>−W</b><i>weight.grd</i> ]</p>
<a name="DESCRIPTION"></a>
<h2>DESCRIPTION</h2>
<p style="margin-left:11%; margin-top: 1em"><b>grdtrend</b>
reads a 2-D grid file and fits a low-order polynomial trend
to these data by [optionally weighted] least-squares. The
trend surface is defined by:</p>
<p style="margin-left:11%; margin-top: 1em">m1 + m2*x +
m3*y + m4*x*y + m5*x*x + m6*y*y + m7*x*x*x + m8*x*x*y +
m9*x*y*y + m10*y*y*y.</p>
<p style="margin-left:11%; margin-top: 1em">The user must
specify <b>−N</b><i>n_model</i>, the number of model
parameters to use; thus, <b>−N</b><i>4</i> fits a
bilinear trend, <b>−N</b><i>6</i> a quadratic surface,
and so on. Optionally, append <b>r</b> to the
<b>−N</b> option to perform a robust fit. In this
case, the program will iteratively reweight the data based
on a robust scale estimate, in order to converge to a
solution insensitive to outliers. This may be handy when
separating a "regional" field from a
"residual" which should have non-zero mean, such
as a local mountain on a regional surface.</p>
<p style="margin-left:11%; margin-top: 1em">If data file
has values set to NaN, these will be ignored during fitting;
if output files are written, these will also have NaN in the
same locations.</p>
<p style="margin-left:11%; margin-top: 1em">No space
between the option flag and the associated arguments.
<i><br>
grdfile</i></p>
<p style="margin-left:22%;">The name of a 2-D binary grid
file.</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">[<b>r</b>]<i>n_model</i>
sets the number of model parameters to fit. Append <b>r</b>
for robust fit.</p></td>
</table>
<a name="OPTIONS"></a>
<h2>OPTIONS</h2>
<p style="margin-left:11%; margin-top: 1em">No space
between the option flag and the associated arguments.</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">Write the
difference (input data - trend) to the file
<i>diff.grd</i>.</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">Write the fitted
trend to the file <i>trend.grd</i>.</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>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−W</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top">If
<i>weight.grd</i> exists, it will be read and used to solve
a weighted least-squares problem. [Default: Ordinary
least-squares fit.] If the robust option has been selected,
the weights used in the robust fit will be written to
<i>weight.grd</i>.</p> </td>
</table>
<a name="REMARKS"></a>
<h2>REMARKS</h2>
<p style="margin-left:11%; margin-top: 1em">The domain of x
and y will be shifted and scaled to [-1, 1] and the basis
functions are built from Legendre polynomials. These have a
numerical advantage in the form of the matrix which must be
inverted and allow more accurate solutions. NOTE: The model
parameters listed with <b>−V</b> are Legendre
polynomial coefficients; they are not numerically equivalent
to the m#s in the equation described above. The description
above is to allow the user to match <b>−N</b> with the
order of the polynomial surface. See <b><A HREF="grdmath.html">grdmath</A></b> if you
need to evaluate the trend using the reported
coefficients.</p>
<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="EXAMPLES"></a>
<h2>EXAMPLES</h2>
<p style="margin-left:11%; margin-top: 1em">To remove a
planar trend from hawaii_topo.grd and write result in
hawaii_residual.grd:</p>
<p style="margin-left:11%; margin-top: 1em"><b>grdtrend</b>
hawaii_topo.grd <b>−N</b>3
<b>−D</b>hawaii_residual.grd</p>
<p style="margin-left:11%; margin-top: 1em">To do a robust
fit of a bicubic surface to hawaii_topo.grd, writing the
result in hawaii_trend.grd and the weights used in
hawaii_weight.grd, and reporting the progress:</p>
<p style="margin-left:11%; margin-top: 1em"><b>grdtrend</b>
hawaii_topo.grd <b>−N</b>10<b>r
−T</b>hawaii_trend.grd
<b>−W</b>hawaii_weight.grd <b>−V</b></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="grdfft.html">grdfft</A></i>(1), <i><A HREF="grdfilter.html">grdfilter</A></i>(1)</p>
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