 <code>dframe</code> is used to find the y-coordinate). The default
value is <code>NULL</code>.</p></dd>


<dt>stratumID</dt>
<dd><p>Character value providing name of the stratum ID variable in
the <code>dframe</code> data frame.  The default value is <code>NULL</code>.</p></dd>


<dt>clusterID</dt>
<dd><p>Character value providing the name of the cluster
(stage one) ID variable in <code>dframe</code>.  Note that cluster
IDs are required for a two-stage sample.  The default value is <code>NULL</code>.</p></