<para>
This filter keeps or removes waypoints based on their proximity to an arc,
which is a series of connected line segments similar to a route or a track
but without any associated data other than the coordinates.  Optionally, it
can move each non-deleted waypoint over the closest segment of the arc.
</para>

<para>
The arc may defined in a file whose name must be provided with the
<option>file</option>, or the tracks or routes that have already been read.  That file contains pairs of coordinates for the
vertices of the arc, one coordinate pair per line.  Comments may be
included by preceding them with a '#' character.  An arc file looks
something like this sample:
</para>

<screen>
# Lima Road/SR3 north of Fort Wayne, Indiana
41.150064468    -85.166207433
41.150064468    -85.165371895
41.149034500    -85.165157318
41.147832870    -85.164771080
41.146631241    -85.164384842
41.144270897    -85.163655281
41.141953468    -85.162882805
</screen>

<para>
An arc file may optionally contain gaps in the arc.  You may specify
such a gap by inserting a line containing "#break" either on a line by
itself or after the coordinates of the starting point of the new arc segment.
</para>

<example xml:id="example_arc_filter">
<title>Using the arc filter</title>
<para>
Assuming the arc above is in a file called
<filename>lima_rd.txt</filename>, the following command line
would include only points within one mile of the section of Lima Road
covered by the arc. The output is reordered by distance to the arc,
closest points first.

</para>
<para><userinput>gpsbabel -i geo -f 1.loc -x arc,file=lima_rd.txt,distance=1 -o mapsend -F 2.wpt</userinput></para>
</example>
