z-axis scaling, whereas <code>area</code> and <code>cubes</code> methods
are. Here are estimates for the relative flat surface we simulated with
<code>smoothness=0.5</code>.</p>
<div class="sourceCode" id="cb4"><pre class="downlit sourceCode r">
<code class="sourceCode R"><span><span class="co"># height variation</span></span>
<span><span class="fu"><a href="../reference/fd.html">fd</a></span><span class="op">(</span><span class="va">surf</span>, method <span class="op">=</span> <span class="st">"hvar"</spa