Trail Milage and GPS and bikes

["Ed & Wendy Hauser" <sisufarm@mmmpcc.org>]

I just got back from riding my odometer equipped bike on a "20" mile loop of
my ride.  These trails have been used for many years, but I have always felt
that they were "short"

I bought a new odometer and calibrated it per the manufacturers
instructions.  I then rode it on a measured course and got agreement to the
0.01 mile over 1.5 miles.  I think this is as good as it gets.  This morning
I went to the park with my bike and GPS.  I was hopeful that with the
"selective availability" turned off, the odometer on the GPS would work
well.  As these trails are along the beautiful St. Croix River, they are
very flat, so no problems with elevation changes should happen.  Much of the
mileage is wooded however.   At this season, the leaves may screen the
signal.  My results were quite interesting.  I carefully avoided using the
front brake, and steered as straight down the trails as possible.  I did
have to zig-zag at times, so my bike mileage should be on the high side.

In the past my GPS has seemed to understate the mileage by about 10%.  Now
on the first half of the loop the bike said 9.49 miles, and the GPS said
9.15 miles.  An error of -3.6%.  Not bad.

Things got interesting on the way back.  On one section (which was going
over the same trail in the opposite direction) the bike said 5.08 miles but
the GPS said 6.98 miles!   By the end of the loop the GPS said I had gone
19.71 miles, but the bike said 18.73 miles.  For a difference of +5.2%.

My conclusion is that while things are better now, my GPS does strange
things with mileage every once in a while.  The accuracy is certainly good
enough for training rides, but if using it to measure a distance ride one
should double check for strange things.  I don't know why my GPS messes up
mileage, because it always locates my position with great precision.

By the way if anyone is going to be in MN Sept 16,17 come to Sisu-Sisu.  I
can at least guarantee accurately measured trails, and a spaghetti dinner.
(an email would get a ride flyer).

Ed and Wendy Hauser
1140 37th St.
Hudson, WI 54016
sisufarm@mmmpcc.org