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<HTML>
<HEAD>
<TITLE>grdtrack</TITLE>
</HEAD>
<BODY>
<H1>grdtrack</H1>
<HR>
<PRE>
<!-- Manpage converted by man2html 3.0.1 -->
grdtrack - Sampling of a 2-D grdfile along 1-D trackline
(a sequence of x,y points)
</PRE>
<H2>SYNOPSIS</H2><PRE>
<B>grdtrack</B> <I>xyfile</I> <B>-G</B><I>grdfile</I> [ <B>-H</B>[<I>nrec</I>] ] [ <B>-L</B>[<I>flag</I>] ] [
<B>-M</B>[<I>flag</I>] ] [ <B>-Q</B> ] [ <B>-R</B><I>west/east/south/north</I> ] [ <B>-S</B> ] [ <B>-V</B>
] [ <B>-Z</B> ] [ <B>-:</B> ] [ <B>-bi</B>[<B>s</B>][<I>n</I>] ] [ <B>-bo</B>[<B>s</B>] ]
</PRE>
<H2>DESCRIPTION</H2><PRE>
<B>grdtrack</B> reads a grdfile and a table (from file or stan
dard 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] or bilinear [<B>-Q</B>] interpolation is used, requir
ing boundary conditions at the limits of the region (see
<B>-L</B><I>flag</I> option).
<I>xyfile</I> 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.
<B>-G</B> <I>grdfile</I> is a 2-D binary grd file with the function
f(x,y).
</PRE>
<H2>OPTIONS</H2><PRE>
No space between the option flag and the associated argu
ments.
<B>-H</B> Input file(s) has Header record(s). Number of
header records can be changed by editing your .gmt
defaults file. If used, <B><A HREF="GMT.html">GMT</A></B> default is 1 header
record.
<B>-L</B> Boundary condition <I>flag</I> may be <I>x</I> or <I>y</I> or <I>xy</I> indi
cating 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).] If no flag is supplied,
it is assumed that the x column contains longi
tudes, which may differ from the region in <B>-R</B> by
[multiples of] 360 degrees [Default assumes no
periodicity].
<B>-M</B> Multiple segment file. Segment separator is a
record beginning with <I>flag</I>. [Default is '>'].
<B>-Q</B> Quick mode. Use bilinear rather than bicubic inter
polation.
<B>-R</B> <I>west,</I> <I>east,</I> <I>south,</I> and <I>north</I> specify the Region of
Append <B>r</B> if lower left and upper right map coordi
nates are given instead of wesn.
<B>-S</B> Suppress the output of interpolated points that
result in NaN values.
<B>-V</B> Selects verbose mode, which will send progress
reports to stderr [Default runs "silently"].
<B>-Z</B> Only write out the sampled z-values [Default writes
all columns].
<B>-:</B> Toggles between (lon,lat) and (lat,lon) input/out
put. [Default is (lon,lat)]
<B>-bi</B> Selects binary input. Append <B>s</B> for single precision
[Default is double]. Append <I>n</I> for the number of
columns in the binary file(s). [Default is 2 input
columns].
<B>-bo</B> Selects binary output. Append <B>s</B> for single preci
sion [Default is double].
</PRE>
<H2>HINTS</H2><PRE>
If an interpolation point is not on a node of the input
grid, then a NaN at any node in the neighborhood surround
ing the point will yield an interpolated NaN. Bicubic
interpolation [default] yields continuous first deriva
tives but requires a neighborhood of 4 nodes by 4 nodes.
Bilinear interpolation [<B>-Q</B>] uses only a 2 by 2 neighbor
hood, but yields only zeroth-order continuity. Use bicubic
when smoothness is important. Use bilinear to minimize the
propagation of NaNs.
</PRE>
<H2>EXAMPLES</H2><PRE>
To sample the file hawaii_topo.grd along the SEASAT track
track_4.xyg (An ASCII table containing longitude, lati
tude, and SEASAT-derived gravity, preceeded by one header
record), try
grdtrack track_4.xyg <B>-G</B>hawaii_topo.grd <B>-H</B> > track_4.xygt
</PRE>
<H2>SEE ALSO</H2><PRE>
<I>gmt</I>(l), <I><A HREF="surface.html">surface</A></I>(l), <I><A HREF="sample1d.html">sample1d</A></I>(l)
</PRE>
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