="../reference/hr.html">hr</a></span><span class="op">(</span><span class="va">surf_z</span><span class="op">)</span><span class="op">)</span></span>
<span><span class="fu"><a href="../reference/hr.html">hr</a></span><span class="op">(</span><span class="va">surf_z</span><span class="op">)</span></span>
<span><span class="co">#&gt; [1] 63</span></span>
<span></span>
<span><span class="co"># height variation</span></span>
<span><span class="fu"><a href="../reference/fd.html">fd</a></span><span class="op">(</