Re: Trail Milage and GPS and bikes

["navion" <navion@lightspeed.net>]

> 9.15 miles.  An error of -3.6%.  Not bad.
>
> 19.71 miles, but the bike said 18.73 miles.  For a difference of +5.2%.

    I have found the same things using my Garmin GPS. (I have the external
ant. and mount it on my helmet)   Plenty good enough for scouting, and rough
measurements, but not as accurate as one should be for a real ride course.
So please those measuring courses, don't totally rely on a GPS. I find them
usually to measure short, making the course long.


> 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.
>
    Lots of reasons that I can think of. Probably more I can't.
1) Internal software attempts to take out random drifting, this causes
winding courses to be measured short.
2) GPS gives data for sea level, horizontal distances. (at an altitude
higher than sea level, the larger radius of the earth gives longer
distances. )  I believe the software does not calibrate for anything above
sea level, I may be wrong. But it definately does not do slopes. Again
causing short measurements.

     For those really into careful measurement, I refer you to the USA Track
and Field manual for measuring running courses. This manual is the bible for
measuring olympic marathons, etc etc here in the USA.  I have on order the
special counter that is the only counter certified to be used to measure
distances. I guess they feel the magnetic counters we have on our bikes can
miss a count and measure short.  The address is.
http://users.hit.net/~bobbau/rrtc/book/#contents

      Those engineers out there are going to love the detail of this manual.

Jim Mitchell