<!--Copyright (C) 1988-2005 by the Institute of Global Environment and Society (IGES). See file COPYRIGHT for more information.--> <h2><b>maskout</b></h2><p> <code>maskout(<i>expr,mask</i>)</code><p> Wherever the <code><i>mask</i></code> values are less than zero, the values in <code><i>expr</i></code> are set to the missing data value. <p> Works with gridded or station data. Where <code><i>mask</i></code> values are positive, the <code><i>expr</i></code> values are not modified. Thus the result of <code>maskout</code> is data with a possibly increased number of missing data values. The <code>maskout</code> function, in spite of its apparant simplicity, is extremely useful. <p> <H3>Usage Notes</H3><P> <h3>Examples</h3><p> <ol> <li>See the Examples for the <a href="gradfuncconst.html"><code>const</code></a> function for a description of using <code>maskout</code> to calculate the percentage of the globe covered by precipitation. <p> <li>The <code>maskout</code> function can be used to cause part of the data to be ignored while doing another calculation. For example, if we have a land-sea mask, where sea values are negative, and we want to take some areal average of a quantity only over land: <p> <dd><code>d aave(maskout(p,mask.2),lon=0,lon=360,lat=0,lat=90) </code><p> <li>People frequently have trouble using a mask grid, because it is often put into a seperate file, and given some arbitrary date/time and level. Thus, it is often necessary to <i>locally override</i> the dimension environment while using the mask grid: <p> <dd><code>d aave(maskout(p,mask.2(t=1)),lon=0,lon=360,lat=0,lat=90) </code><p> would probably be how Example 2 would have to be expressed in order to work, with the local override of <code>t=1</code> specified on the mask data. See the documentation on how GrADS evaluates expressions within the dimension environment for more information.