Re: Most confusing bike route (long and rambling)
- From: "Buck" <bicyclebuck@xxxxxxxxx>
- Date: 29 Sep 2005 07:55:48 -0700
Dave Vandervies wrote:
> So, consumer grade or not, if the measurements and calculations a
> cyclometer needs can't be done to .5%-1% error AS AN ABSOLUTE UPPER
> BOUND, there's no excuse other than shoddy worksmanship somewhere along
> the line. Hopefully it's in the wheel measurement or the calibration
> ride to account for wheel deformation, since that's where the user can
> re-do things, and not in the cyclometer itself.
One thing that you either forgot to add or are not aware of is the
error in GPS due to elevational changes. As I stated before, the
vertical accuracy of GPS (no matter which grade you are using) is
roughly three times the horizontal. Also note that the distances
reported by GPS may be planimetric (it depends on your settings)
instead of accounting for 3-dimensional terrain. This can introduce
additional total-distance errors.
As an example, let me share some of the results of a calibration test I
performed on 14 Trimble GeoExplorer 3 mapping-grade GPS units. For the
point-location test, I placed all of the units directly adjacent to
one-another (arranged all within a 2 ft. circle) and let them collect
500 points (one per second). The conditions were perfect with a clear,
unobstructed sky. The Position Dilution of Precision (PDOP) was set not
to exceed 6 (a generally accepted value).
The GPS units report their horizontal and vertical precision estimates
based on the average calculated position over the 500 location
recordings. These were post-processed to correct for errors using a
base station that was less than 100 meters from the test site. The
horizontal precisions reported by the GPS units ranged from 2.3 to 3.1
meters. The vertical precisions ranged from 4.3 to 5.3 meters.
But here's the kicker - the horizontal RMS error from the locations
reported by all of the units was 6.88 meters. The vertical positions
reported ranged from 76.889 to 107.681 meters. That is a vertical error
range of 30.792 meters.
So, the locations reported are accurate within an egg-shaped ellipsoid
with a horizontal axis of 6.88 meters and a vertical axis of 30.792
meters.
It would be fine if the precision errors were consistent and you only
used one gps unit (accuracy being ignored), but precision errors change
as you change your true position, the amount of overhead cover (trees,
buildings, etc.), the number of satellites available, the time of day,
and even the amount of solar activity.
The only way survey-grade GPS can achieve its high accuracies is with a
site occupation time of at least two hours (with contant data
collection), carrier-phase processing (instead of code like most
recreational and mapping grade units use), and post or real-time error
correction.
A cyclometer, since it is only counting wheel revolutions, measures
true surface distance instead of estimating surface position and
interpolating distance from a series of reported positions. You are
right that there may be path errors with a cyclometer, but I think that
you properly assessed that they would be very minimal, likely
non-significant.
The biggest benefits to using GPS are the ability to map your travels,
estimate your distance, and estimate the amount of vertical distance
you covered. These measurements are hard to get from the typical
cyclecomputer!
-Buck
.
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