Sullivan D and Unwin D (2010) Geographic Information Analysis, 2nd Edition, Chapter 7. John Wiley &amp; Sons: New Jersey.</p>
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<div id="preliminaries-26" class="section level2 hasAnchor" number="28.4">
<h2><span class="header-section-number">28.4</span> Preliminaries<a href="activity-13-area-data-v.html#preliminaries-26" class="anchor-section" aria-label="Anchor link to header"></a></h2>
<p>Begin by restarting your <code>R</code> session, or at least by clearing the working space to make sure that you