<p>Hovering over and clicking on alluvia are more difficult because the shapes of the alluvia are more complex. The default shape of the polygons includes an <code>xspline</code> curve drawn using the <strong>grid</strong> package. We need to manually reconstruct the coordinates of the polygons, then use <code>sp::pointInPolygon()</code> to detect which, if any, polygons the cursor is over.</p>
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