<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>
The units may be specified by appending a suffix to the supplied number:
</para>
<simplelist type="vert">
<member>'m' for meters, e.g. 3500.0m</member>
<member>'ft' or 'feet' for feet, e.g. 11483ft</member>
<member>'k' or 'km' for kilometers, e.g 3.5000km</member>
<member>'nm' for nautical miles, e.g. 1.8898nm</member>
<member>'mi' for miles, e.g. 2.1748mi</member>
<member>'fa' for fathoms, e.g. 1913.82fa</member>
</simplelist>
<para>
If no units are specified, the units are assumed to be miles.
</para>
