<para>
This option is not required, but if it is not specified the distance
defaults to zero miles, which isn't very useful.
</para>
<para>
This option specifies the maximum distance a point may be from the arc
without being discarded.  Points that are closer to the arc are kept, while
points that are further away are discarded.
</para>
<para>
Distances may be specified in miles (3M) or kilometers (5K).  If no units
are specified, the distance is assumed to be in miles.
</para>
