<!-- Creator : groff version 1.19.2 --> <!-- CreationDate: Wed Jul 14 08:43:30 2010 --> <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta name="generator" content="groff -Thtml, see www.gnu.org"> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <meta name="Content-Style" content="text/css"> <style type="text/css"> p { margin-top: 0; margin-bottom: 0; } pre { margin-top: 0; margin-bottom: 0; } table { margin-top: 0; margin-bottom: 0; } </style> <title>GRDTREND</title> </head> <body bgcolor="#ffffff"> <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> <hr> </body> </html>