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<title>GRDTRACK</title>
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<h1 align="center">GRDTRACK</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="#ASCII FORMAT PRECISION">ASCII FORMAT PRECISION</a><br>
<a href="#GRID FILE FORMATS">GRID FILE FORMATS</a><br>
<a href="#HINTS">HINTS</a><br>
<a href="#EXAMPLES">EXAMPLES</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">grdtrack
− Sampling of a 2-D grid file along 1-D trackline (a
sequence of x,y points)</p>
<h2>SYNOPSIS
<a name="SYNOPSIS"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>grdtrack</b>
<i>xyfile</i> <b>−G</b><i>grdfile</i> [
<b>−H</b>[<b>i</b>][<i>nrec</i>] ] [
<b>−L</b><i>flag</i> ] [
<b>−Q</b>[<b>b</b>|<b>c</b>|<b>l</b>|<b>n</b>][[<b>/</b>]<i>threshold</i>]
] [
<b>−R</b><i>west</i>/<i>east</i>/<i>south</i>/<i>north</i>[<b>r</b>]
] [ <b>−S</b> ] [ <b>−V</b> ] [ <b>−Z</b>
] [ <b>−:</b>[<b>i</b>|<b>o</b>] ] [
<b>−b</b>[<b>i</b>|<b>o</b>][<b>s</b>|<b>S</b>|<b>d</b>|<b>D</b>[<i>ncol</i>]|<b>c</b>[<i>var1</i><b>/</b><i>...</i>]]
] [ <b>−f</b>[<b>i</b>|<b>o</b>]<i>colinfo</i> ] [
<b>−m</b>[<b>i</b>|<b>o</b>][<i>flag</i>] ]</p>
<h2>DESCRIPTION
<a name="DESCRIPTION"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>grdtrack</b>
reads a grid file (or a Sandwell/Smith IMG file) and a table
(from file or standard input) with (x,y) positions in the
first two columns (more columns may be present). It
interpolates the grid at the positions in the table and
writes out the table with the interpolated values added as a
new column. A bicubic [Default], bilinear, B-spline or
nearest-neighbor (see <b>−Q</b>) interpolation is
used, requiring boundary conditions at the limits of the
region (see <b>−L</b>).</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="9%">
<p><i>xyfile</i></p></td>
<td width="2%"></td>
<td width="78%">
<p>This is an ASCII (or binary, see <b>−b</b>) file
where the first 2 columns hold the (x,y) positions where the
user wants to sample the 2-D data set.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="9%">
<p><b>−G</b></p></td>
<td width="2%"></td>
<td width="78%">
<p><i>grdfile</i> is a 2-D binary grid file with the
function f(x,y). If the specified grid is in Sandwell/Smith
Mercator format you must append a comma-separated list of
arguments that includes a scale to multiply the data
(usually 1 or 0.1), the mode which stand for the following:
(0) Img files with no constraint code, returns data at all
points, (1) Img file with constraints coded, return data at
all points, (2) Img file with constraints coded, return data
only at constrained points and NaN elsewhere, and (3) Img
file with constraints coded, return 1 at constraints and 0
elsewhere, and optionally the max latitude in the IMG file
[80.738]. (See GRID FILE FORMAT below.)</p></td></tr>
</table>
<h2>OPTIONS
<a name="OPTIONS"></a>
</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="4%">
<p style="margin-top: 1em"><b>−H</b></p></td>
<td width="7%"></td>
<td width="78%">
<p style="margin-top: 1em">Input file(s) has header
record(s). If used, the default number of header records is
<b><A HREF="gmtdefaults.html#N_HEADER_RECS">N_HEADER_RECS</A></b>. Use <b>−Hi</b> if only input
data should have header records [Default will write out
header records if the input data have them]. Blank lines and
lines starting with # are always skipped.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−L</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Boundary condition <i>flag</i> may be <i>x</i> or
<i>y</i> or <i>xy</i> indicating data is periodic in range
of x or y or both set by <b>−R</b>, or <i>flag</i> may
be <i>g</i> indicating geographical conditions (x and y are
lon and lat). [Default uses "natural" conditions
(second partial derivative normal to edge is zero) unless
the grid is automatically recognised as periodic.]</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−Q</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Quick mode, use bilinear rather than bicubic
interpolation [Default]. Alternatively, select the
interpolation mode by adding <b>b</b> for B-spline
smoothing, <b>c</b> for bicubic interpolation, <b>l</b> for
bilinear interpolation or <b>n</b> for nearest-neighbor
value. Optionally, append <i>threshold</i> in the range
[0,1]. This parameter controls how close to nodes with NaN
values the interpolation will go. E.g., a <i>threshold</i>
of 0.5 will interpolate about half way from a non-NaN to a
NaN node, whereas 0.1 will go about 90% of the way, etc.
[Default is 1, which means none of the (4 or 16) nearby
nodes may be NaN]. <b>−Q0</b> will just return the
value of the nearest node instead of interpolating. This is
the same as using <b>−Qn</b>.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−R</b></p></td>
<td width="7%"></td>
<td width="78%">
<p><i>xmin</i>, <i>xmax</i>, <i>ymin</i>, and <i>ymax</i>
specify the Region of interest. For geographic regions,
these limits correspond to <i>west, east, south,</i> and
<i>north</i> and you may specify them in decimal degrees or
in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append <b>r</b> if
lower left and upper right map coordinates are given instead
of w/e/s/n. The two shorthands <b>−Rg</b> and
<b>−Rd</b> stand for global domain (0/360 and
-180/+180 in longitude respectively, with -90/+90 in
latitude). Alternatively, specify the name of an existing
grid file and the <b>−R</b> settings (and grid
spacing, if applicable) are copied from the grid. For
calendar time coordinates you may either give (a) relative
time (relative to the selected <b><A HREF="gmtdefaults.html#TIME_EPOCH">TIME_EPOCH</A></b> and in the
selected <b><A HREF="gmtdefaults.html#TIME_UNIT">TIME_UNIT</A></b>; append <b>t</b> to
<b>−JX</b>|<b>x</b>), or (b) absolute time of the form
[<i>date</i>]<b>T</b>[<i>clock</i>] (append <b>T</b> to
<b>−JX</b>|<b>x</b>). At least one of <i>date</i> and
<i>clock</i> must be present; the <b>T</b> is always
required. The <i>date</i> string must be of the form
[-]yyyy[-mm[-dd]] (Gregorian calendar) or yyyy[-Www[-d]]
(ISO week calendar), while the <i>clock</i> string must be
of the form hh:mm:ss[.xxx]. The use of delimiters and their
type and positions must be exactly as indicated (however,
input, output and plot formats are customizable; see
<b><A HREF="gmtdefaults.html">gmtdefaults</A></b>).</p> </td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−S</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Suppress the output of interpolated points that result
in NaN values.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−V</b></p></td>
<td width="7%"></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="4%">
<p><b>−Z</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Only write out the sampled z-values [Default writes all
columns].</p> </td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−:</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Toggles between (longitude,latitude) and
(latitude,longitude) input/output. [Default is
(longitude,latitude)].</p> </td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−bi</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Selects binary input. Append <b>s</b> for single
precision [Default is <b>d</b> (double)]. Uppercase <b>S</b>
or <b>D</b> will force byte-swapping. Optionally, append
<i>ncol</i>, the number of columns in your binary input file
if it exceeds the columns needed by the program. Or append
<b>c</b> if the input file is netCDF. Optionally, append
<i>var1</i><b>/</b><i>var2</i><b>/</b><i>...</i> to specify
the variables to be read. [Default is 2 input columns].</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−bo</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Selects binary output. Append <b>s</b> for single
precision [Default is <b>d</b> (double)]. Uppercase <b>S</b>
or <b>D</b> will force byte-swapping. Optionally, append
<i>ncol</i>, the number of desired columns in your binary
output file. [Default is one more than input].</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−f</b></p></td>
<td width="7%"></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>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−m</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Multiple segment file(s). Segments are separated by a
special record. For ASCII files the first character must be
<i>flag</i> [Default is ’>’]. For binary
files all fields must be NaN and <b>−b</b> must set
the number of output columns explicitly. By default the
<b>−m</b> setting applies to both input and output.
Use <b>−mi</b> and <b>−mo</b> to give separate
settings to input and output.</p></td></tr>
</table>
<h2>ASCII FORMAT PRECISION
<a name="ASCII FORMAT PRECISION"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">The ASCII
output formats of numerical data are controlled by
parameters in your .gmtdefaults4 file. Longitude and
latitude are formatted according to
<b><A HREF="gmtdefaults.html#OUTPUT_DEGREE_FORMAT">OUTPUT_DEGREE_FORMAT</A></b>, whereas other values are
formatted according to <b><A HREF="gmtdefaults.html#D_FORMAT">D_FORMAT</A></b>. Be aware that the
format in effect can lead to loss of precision in the
output, which can lead to various problems downstream. If
you find the output is not written with enough precision,
consider switching to binary output (<b>−bo</b> if
available) or specify more decimals using the
<b><A HREF="gmtdefaults.html#D_FORMAT">D_FORMAT</A></b> setting.</p>
<h2>GRID FILE FORMATS
<a name="GRID FILE FORMATS"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b><A HREF="GMT.html">GMT</A></b> is
able to recognize many of the commonly used grid file
formats, as well as the precision, scale and offset of the
values contained in the grid file. When <b><A HREF="GMT.html">GMT</A></b> needs a
little help with that, you can 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. 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. 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>HINTS
<a name="HINTS"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">If an
interpolation point is not on a node of the input grid, then
a NaN at any node in the neighborhood surrounding the point
will yield an interpolated NaN. Bicubic interpolation
[default] yields continuous first derivatives but requires a
neighborhood of 4 nodes by 4 nodes. Bilinear interpolation
[<b>−Q</b>] uses only a 2 by 2 neighborhood, but
yields only zeroth-order continuity. Use bicubic when
smoothness is important. Use bilinear to minimize the
propagation of NaNs, or lower <i>threshold</i>.</p>
<h2>EXAMPLES
<a name="EXAMPLES"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">To sample the
file hawaii_topo.grd along the SEASAT track track_4.xyg (An
ASCII table containing longitude, latitude, and
SEASAT-derived gravity, preceded by one header record):</p>
<p style="margin-left:11%; margin-top: 1em"><b>grdtrack</b>
track_4.xyg <b>−G</b> hawaii_topo.grd <b>−H</b>
> track_4.xygt</p>
<p style="margin-left:11%; margin-top: 1em">To sample the
Sandwell/Smith IMG format file topo.8.2.img (2 minute
predicted bathymetry on a Mercator grid) along the lon,lat
coordinates given in the file cruise_track.xy, try</p>
<p style="margin-left:11%; margin-top: 1em"><b>grdtrack</b>
cruise_track.xy <b>−G</b> topo.8.2.img,1,1 >
obs_and_predicted.d</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="surface.html">surface</A></i>(1), <i><A HREF="sample1d.html">sample1d</A></i>(1)</p>
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