<para>
This option is required.
</para>
<para>
This option specifies the maximum distance a point may be from the central
point in order to remain in the dataset.  Points closer than this distance
will be kept and points further away will be removed (unless the 
<option>exclude</option> option is specified.)
</para>
<para>
Distances may be expressed in miles (3M) or kilometers (4K).  If no units
are provided, the distance is assumed to be in miles.
</para>
