<!--Copyright (C) 1988-2005 by the Institute of Global Environment and Society (IGES). See file COPYRIGHT for more information.-->

<h2>Dimension Environment</h2>
<p>

The data set is always viewed by GrADS as a generalized 5-D (6-D
if you include variables) array located in physical space (lon,
lat, lev, time, ensemble), even if it is in reality a subset of a 5-D
space.<p>

The current dimension environment describes what part of the data
set you want to work with. Expressions are evaluated with respect
to the dimension environment (which allows for simplicity in the
expression syntax), and the final display will be determined by
the dimension environment.  Thus, the dimension environment is a
GrADS concept that is important to understand.<p>

The dimension environment is manipulated by the user by entering
one of the following set commands:<p>

<dd><code><a href="gradcomdsetlatlonlevtimeens.html">set lat|lon|lev|time|ens</a> <i>val1
&lt;val2&gt;</i></code>
  <p>

This set command sets one dimension of the dimension environment
using world coordinates.<p>

Alternatively:<p>

<dd><code><a href="gradcomdsetxyzte.html">set x|y|z|t|e</a><i> val1
&lt;val2&gt;</i></code>
  <p>

This sets one dimension of the dimension environment using grid
coordinates.  You may use whatever coordinates are convenient to
you.  Issuing <code><a href="gradcomdsetlatlonlevtimeens.html">set lon</a></code> is
equivalent to issuing <code><a href="gradcomdsetxyzte.html">set x</a></code>, both
set the x dimension.  The difference is only the units you wish
to enter the command in.<p>

When you enter just one value, that dimension is said to be
"fixed".  When you enter two values, that dimension is said to be
"varying".  The combination of fixed and varying dimensions
defines the dimension environment.<p>

Examples:<p>
<ul>
<code><a href="gradcomdsetlatlonlevtimeens.html">set lon</a> -180
0</code>&nbsp;&nbsp;&nbsp;&nbsp;  sets
longitude to vary from 180W to 0<br>
<code><a href="gradcomdsetlatlonlevtimeens.html">set lat</a> 0
90</code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; sets
latitude to vary from the equator to 90N<br>
<code><a href="gradcomdsetlatlonlevtimeens.html">set lev</a>
500</code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; sets
the level to 500mb - a fixed dimension<br> 
<code><a href="gradcomdsetxyzte.html">set t</a>
1</code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
sets time to the first time in the data set--using grid
coordinates in this case. Time is now a fixed dimension</ul><p>

When <b>all</b> dimensions are fixed, you are referring to a <b>single</b> data
point.<p>

When <b>one</b> dimension is varying, you are referring to a <b>1-D "slice"</b>
through the data set.<p>

When <b>two</b> dimensions are varying, you are referring to a <b>2-D
"slice"</b> through the data set.
<p>

When <b>three or more</b> dimension vary, GrADS interprets this as a <b>sequence of
2-D slices</b>.
<p>

<b>An important note:</b>  When you specificy the dimension environment in terms of  grid
coordinates, they are always converted to world coordinates. 
This conversion  requires some knowledge of what scaling is in
use for grid to world conversions.  The scaling that is used in
all cases (except one) is the scaling of the <i>default file</i>. The
exception is when you supply a dimension expression within a
variable specification, which will be covered later.