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Jim & others interested:
I have gotten to the bottom of the discussion and our differences on
accuracy.
What happens is the Garmin unit feeds the data through what is called a
KALMAN FILTER. This takes into account the direction and velocity of motion
and affords a bit of predictability as to the next point.
The end result is that the distance covered, i.e. the odometer effect is
quite accurate. Almost as accurate as the differentially correct GPS data.
However the ability to plot an accurate map route of travel is somewhat
limited. From what I have learned the plotted route could be off in the
range of 50 meters at any/all points on the route.
The good news is that this is a very good method of checking ride lengths,
if plotted it will equal or better any hand drawn route on a USGS Quad.. The
bad news is the route should not be published or used for establishing
routes on public property such as BLM or USFS or State lands. The only way
to do that is with differentially corrected data and then there is no point
that can ever be questioned.
So, all you potential trail measurers get to work. A word of caution though,
do a double measurement BEFORE you ever question a ride mileage. BE SURE OR
BE PREPARED TO BE SORRY
Bob Morris
Morris Endurance Enterprises
Boise, ID
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