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<!--Copyright (C) 1988-2005 by the Institute of Global Environment and Society (IGES). See file COPYRIGHT for more information.-->
<html>
<head>
<title>Cross Sections</title>
</head>
<body text="#000000" bgcolor="e0f0ff">
<h2>Drawing Arbitrary Cross Sections</h2>
<p>
Drawing arbitrary vertical cross sections based on a collection of
station data profiles involves transforming station data (scattered
observations) into gridded data so as to take advantage of the GrADS
grid display and analysis features.
<p>
The first step is to form a collection of 1-D data (Z or T
varying). The <a href="gradcomdcollect.html"><code>collect</code></a>
command saves station data profiles or time series in memory as a
set. The 1-D data may be either real station data or
gridded data converted to station data using
<a href="gradfuncgr2stn.html"><code>gr2stn</code></a>.
<p>
The second stop is to convert the collection of station data into
a grid for display or analysis purposes. This is accomplished by
the new function <a href="gradfunccoll2gr.html"><code>coll2gr</code></a>.
<p>
<a href="gradfunccoll2gr.html"><code>coll2gr</code></a> does not yet
support time slices; currently, it will only work when the collection
of stations is a collection of vertical profiles.
<p>
<a href="gradfunccoll2gr.html"><code>coll2gr</code></a> produces an
output grid that varies in X and Z; the dimension environment used
when <a href="gradfunccoll2gr.html"><code>coll2gr</code></a> is
invoked must also be X and Z varying. The X axis of the output grid
will contain the equally spaced station profiles and will span the
range of the current X dimension environment. The Z axis of the output
grid will span the range of the current Z dimension environment and
will have either the specified number of levels or a union of the
levels. Data points outside of the range of levels will be used for
interpolating to within the range if appropriate.
<p>
The X axis of the output grid from <a
href="gradfunccoll2gr.html"><code>coll2gr</code></a> is artificial in
terms of the world coordinates -- it doesn't really represent
longitudes. A way to completely control the labelling of the display
output is provided:
<p>
<ul>
<code>
<a href="gradcomdsetxlabs.html">set xlabs</a> lab1 | lab2 | lab3 ...<br>
<a href="gradcomdsetylabs.html">set ylabs</a> lab1 | lab2 | lab3 ...<br>
</code>
</ul>
<p>
Each label string may include blanks. The labels will be plotted
equally spaced along the indicated axis. Spacing can be
modified by adding blank strings:
<p>
<ul>
<code><a href="gradcomdsetxlabs.html">set xlabs</a> | | | | lab1 | ...<br>
</code>
</ul>
<p>
Here is a sample script written by M. Fiorino that uses these features:
<p>
<code>
*********************************************************************<br>
* The following lines will display an arbitrary X section<br>
* from one specified point to another. <br>
*<br>
* lon1 is the westernmost longitude point<br>
* lon2 is the easternmost longitude point<br>
* lat1 is the latitude that corresponds to lon1<br>
* lat2 is the latitude that corresponds to lon2<br>
*<br>
* The loop is used to interpolate between points in<br>
* the arbitrary cross section. This code will plot<br>
* any cross section as long as you specify the points. <br>
* My code plots cross sections of PV after I calculated<br>
* PV on 11 pressure surfaces. I have another script<br>
* that plots cross sections of potential temperature, and<br>
* the code is very similar to this, except theta is substituted<br>
* for PV.<br>
*<br>
* Many thanks to Brian Doty at COLA for his help with this code.<br>
*<br>
********************************************************************<br>
<br>
'open pv.ctl'<br>
'set grads off'<br>
'set zlog on'<br>
'set x 1'<br>
'set y 1'<br>
'set lev 1000 100'<br>
lon1 = -95.0<br>
lon2 = -90.0<br>
lat1 = 55.0<br>
lat2 = 15.0<br>
lon = lon1<br>
'collect 1 free'<br>
while (lon <= lon2)<br>
lat = lat1 + (lat2-lat1)*(lon-lon1) / (lon2-lon1)<br>
'collect 1 gr2stn(pv,'lon','lat')'<br>
lon = lon + 1<br>
endwhile<br>
<br>
'set x 14 16'<br>
'set xaxis 'lon1' 'lon2<br>
'set clab on'<br>
'set gxout shaded'<br>
'set clevs 0 .5 15'<br>
'set ccols 0 0 7 0'<br>
'd coll2gr(1,-u)'<br>
'set gxout contour' <br>
'set cint .5'<br>
'd coll2gr(1,-u)'<br>
</code>
</body>
</html>
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