<para>
This option computes a value for the GPS speed at each trackpoint.
This is most useful with trackpoints from formats that don't support
speed information or for trackoints synthesized by the 
<link linkend="filter_interpolate">interpolate</link>
filter.  The speed at each trackpoint is the average speed from the
previous trackpoint (distance divided by time).  The first trackpoint 
in each track is assigned a speed of "unknown."
</para>
