col class="desc" /></colgroup>
    <tr>
      <th>polygonlayer</th>
      <td><p>A SpatialPolygonsDataFrame containing zones from which
the summary statistics for the catchment variable will be calculated.
Smaller polygons will increase the accuracy of the results.</p></td>
    </tr>
    <tr>
      <th>targetlayer</th>
      <td><p>A SpatialPolygonsDataFrame, SpatialLinesDataFrame,
SpatialPointsDataFrame, SpatialPolygons, SpatialLines or SpatialPoints
object containing the specifications of the facility for