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<td><h1>Basic Concept of Operation</h1>
When you have successfully installed and started GrADS, you'll be
confronted with two windows -- a terminal window with a
prompt (ga->), and a resizable
window (black background by default) where graphics are
displayed.
<p> GrADS commands are entered in the terminal window and the
response from GrADS is either graphics in the graphics window or
text in the terminal window. The three fundamental GrADS
commands: </p>
<ol>
<li><code>open</code> open or make available to GrADS a data file
with
either gridded or station data </li>
<li><code>d</code> display a GrADS
"expression" (e.g., a slice of data) </li>
<li><code>set</code> manipulate the "what" "where" and
"how" of data
display</li>
</ol>
<p> The GrADS "expression," or what you want to look at, can be as
simple as a variable in the data file that was opened, e.g., <code>d
slp</code> or an arithmetic or GrADS function operation on the data,
e.g., <code>d slp/100</code> or <code>d mag(u,v)</code> where mag is a GrADS intrinsic
function.</p>
<p> The "where" of data display is called the "dimension environment"
and defines which part, chunk or "hyperslab" of the 5-D
geophysical space (lon,lat,level,time,ens) is displayed. The
dimension environment is manipulated through the <a href="commands.html#setdimension">set</a> command and
is controlled in either grid coordinates (x,y,z,t,e indices) or <i>world</i> coordinates (<code>lon, lat, lev, time</code>, ens).</p>
<p> The "what" and "how" of display is also controlled by the <a href="commands.html"><code>set</code></a> command and
includes both graphics methods (e.g., contours, streamlines) and
data (e.g., <code>d</code> to a file).</p>
<p> GrADS graphics can be written to a file (i.e., <code><a
href="gradcomdenableprint.html">enable print</a> <i>filename</i></code> and <a
href="gradcomdprint.html"><code>print</code></a>)
and then converted to postscript for
printing
and/or conversion to other image formats.</p>
<p> In addition, GrADS includes graphic primitives (e.g., lines and
circles) and basic labelling through the <a href="commands.html#draw"><code>draw
command</code></a>.</p>
<p> The <code><a href="gradcomdquery.html">q</a></code> or <a href="gradcomdquery.html"><code>query</code></a> command is used to get information from GrADS
such
as which files are opened and even statistics. </p></td>
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<h1> </h1>
