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<H1><A NAME="SECTION0014140000000000000000"></A>
<A NAME="11280"></A>
<BR>
6.14 Gridding of data and trend surfaces
</H1>
<P>
This example shows how one goes from randomly spaced data
points to an evenly sampled surface. First we plot the
distribution and values of our raw data set (table 5.11
from example 12). We choose an equidistant grid and run
<A NAME="tex2html420"
HREF="../blockmean.html"><I><B>blockmean</B></I></A><A NAME="11311"></A> which preprocesses the data to avoid aliasing.
The dashed lines indicate the logical blocks used by
<A NAME="tex2html421"
HREF="../blockmean.html"><I><B>blockmean</B></I></A><A NAME="11320"></A>; all points inside a given bin will be averaged.
The logical blocks are drawn from a temporary file we make on
the fly within the shell script. The processed data is then
gridded with the <A NAME="tex2html422"
HREF="../surface.html"><I><B>surface</B></I></A><A NAME="11329"></A> program and contoured every 25
units. A most important point here is that <A NAME="tex2html423"
HREF="../blockmean.html"><I><B>blockmean</B></I></A><A NAME="11338"></A>,
<A NAME="tex2html424"
HREF="../blockmedian.html"><I><B>blockmedian</B></I></A><A NAME="11347"></A>, or <A NAME="tex2html425"
HREF="../blockmode.html"><I><B>blockmode</B></I></A><A NAME="11356"></A> should always be run
prior to running <A NAME="tex2html426"
HREF="../surface.html"><I><B>surface</B></I></A><A NAME="11365"></A>, and both of these steps must use the same
grid interval. We use <A NAME="tex2html427"
HREF="../grdtrend.html"><I><B>grdtrend</B></I></A><A NAME="11374"></A> to fit a bicubic trend
surface to the gridded data, contour it as well, and sample
both gridded files along a diagonal transect using <A NAME="tex2html428"
HREF="../grdtrack.html"><I><B>grdtrack</B></I></A><A NAME="11383"></A>.
The bottom panel compares the gridded (solid line) and bicubic
trend (dashed line) along the transect using <A NAME="tex2html429"
HREF="../psxy.html"><I><B>psxy</B></I></A><A NAME="11392"></A>
(Figure <A HREF="node87.html#fig:GMT_example_14">6.14</A>):
<P>
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<PRE>
gmtset GRID_PEN 0.25pta
psxy table_5.11 -R0/7/0/7 -JX3.06i/3.15i -B2f1WSNe -Sc0.05i -G0 -P -K -Y6.45i >! example_14.ps
$AWK '{printf "%g %s 6 0 0 5 %g\n", $1+0.08, $2, $3}' table_5.11 | pstext -R -JX -O -K -N \
>> example_14.ps
blockmean table_5.11 -R0/7/0/7 -I1 >! mean.xyz
psbasemap -R0.5/7.5/0.5/7.5 -JX -O -K -B0g1 -X3.25i >> example_14.ps
psxy -R0/7/0/7 -JX -B2f1eSNw mean.xyz -Ss0.05i -G0 -O -K >> example_14.ps
$AWK '{printf "%g %s 6 0 0 5 %g\n", $1+0.1, $2, $3}' mean.xyz | pstext -R -JX -O -K -W255o \
-C0.01i/0.01i -N >> example_14.ps
surface mean.xyz -R -I1 -Gdata.grd
grdcontour data.grd -JX -B2f1WSne -C25 -A50 -G3i/10 -S4 -O -K -X-3.25i -Y-3.55i >> example_14.ps
psxy -R -JX mean.xyz -Ss0.05i -G0 -O -K >> example_14.ps
grdtrend data.grd -N10 -Ttrend.grd
grdcontour trend.grd -JX -B2f1wSne -C25 -A50 -G3i/10 -S4 -O -K -X3.25i >> example_14.ps
project -C0/0 -E7/7 -G0.1 -Fxy >! track
psxy -R -JX track -W1pto -O -K >> example_14.ps
grdtrack track -Gdata.grd | cut -f3,4 >! data.d
grdtrack track -Gtrend.grd | cut -f3,4 >! trend.d
psxy `minmax data.d trend.d -I0.5/25` -JX6.3i/1.4i data.d -W1p -O -K -X-3.25i -Y-1.9i -B1/50WSne \
>> example_14.ps
psxy -R -JX trend.d -W0.5pta -O -U"Example 14 in Cookbook" >> example_14.ps
\rm mean.xyz track *.grd *.d .gmt*
</PRE>
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<DIV ALIGN="CENTER"><A NAME="fig:GMT_example_14"></A><A NAME="11451"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 6.14:</STRONG>
Gridding of data and trend surfaces</CAPTION>
<TR><TD><IMG
WIDTH="401" HEIGHT="527" BORDER="0"
SRC="img117.gif"
ALT="\begin{figure}\centering\epsfig{figure=eps/GMT_example_14.eps}\end{figure}"></TD></TR>
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<ADDRESS>
Paul Wessel
2001-04-18
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