d with a prefix
of "o_" and "d_".</p></td>
    </tr>
    <tr>
      <th>FUN</th>
      <td><p>Function to use on aggregation. Default is sum.</p></td>
    </tr>
    <tr>
      <th>prop_by_area</th>
      <td><p>Boolean value indicating if the values should be
proportionally adjusted based on area. Default is TRUE unless FUN = mean.</p></td>
    </tr>
    <tr>
      <th>digits</th>
      <td><p>The number of digits to use when proportionally adjusting
values based on area. Default is the value of getOption("