Garmin Edge 305 or Edge 500 ??

On Fri, 16 Jul 2010 15:16:31 -0600, wrote:

On Fri, 16 Jul 2010 13:29:14 -0700 (PDT), Jay Beattie
wrote:

On Jul 15, 2:57*pm, wrote:
On Thu, 15 Jul 2010 22:50:56 +0200, Andrew Price
wrote:

On Thu, 15 Jul 2010 14:55:41 +0000 (UTC), David Scheidt
wrote:

You know, the circumfrence of my tires simply doesn't change that
much. *When it does, i stop and fix the flat. *The accuracy of
counting wheel revolutions is greater than that of GPS.

Indeed it is. *To the point where, over my regular 41km training
circuit, a small increase in the normally constant distance measured
is a sure indication that tyre pressure has dropped.

Dear Andrew,

I see the same thing on my even shorter ~15-mile daily ride.

Whenever my odometer reading leaps up a whole 0.020 miles, it's time
to pump my tires and to inspect them with a suspicious glare, in case
a goathead thorn has punctured a Slime tube.

What I don't get is how you can detect tiny errors in accuracy when
you don't ride your daily ride on a measure line. I can go to the
Alpenrose velodrome http://www.flickr.com/photos/cathrund/2690695499/
http://www.flickr.com/photos/brussel...7600747306496/
and ride the blue line or the red line and do different distances.
Lou's Garmin could be entirely accurate -- he may just cut a few more
corners going through the Dolomites on one ride than or another.-- Jay
Beattie.

Dear Jay,

I'm not sure what you're saying, but . . .

When I pump my tires, the measured distance immediately goes up.
^^
. . . er, down

As the tires slowly lose pressure, the measured distance drifts down,
^^^^
. . . er, up
with occasional slight upward spikes for large upward temperature
^^^^^^
er, downward for hotter weather
(the headwinds & traffic are upward)
swings, for days with really bad headwinds that require downshifting

(and more arcing), or when traffic causes me to go longer at the
U-turn on the highway.

When I pump my tires, the measured distance goes up again.
^^
. . . er, down

For what it's worth, the red and blue lines on that velodrome are
considerably farther apart than the line on my daily ride can usually
vary. I ride on the right shoulder of a highway, on the right of a
two-lane no-shoulder road, and on the right of an 8-foot asphalt path.

From what others have pointed out, it seems unlikely than any GPS
odometer can be "entirely accurate" for a real ride.

A GPS samples once (or perhaps twice?) per second, is subject to
circular errors on the order of 15 feet, cuts out unpredictably
without a clear sky view, and apparently doesn't handle elevation
changes as well as horizontal movements.

It sounds as if a GPS measurement's accuracy and repeatability will
improve with a straighter, flatter route with an unobstructed
horizon--perfect for western Kansas.

But it still seems ridiculously accurate and repeatable, if not quite
as accurate and repeatable as a wheel odometer.

If Garmin does indeed prefer the wheel odometer distance to the GPS
part of its system, as one post claims, then it's clear which method
is more repeatable and accurate.

Cheers,

Carl Fogel


Aaargh! When will I learn to proof-read and check that I haven't
gotten things backward?

When the tire is pumped up again, the odometer reading typically goes
back down, not up--I got things reversed.

The pumped-up tire is a tiny bit larger because the contact patch
shrinks a little, reducing the endless shortcut in the otherwise
circular tire.

Over the same distance, the slightly larger tire spins fewer times, so
the odometer shows 999 spins/miles/kilometers/furlongs instead of 1000
spins/miles/kilometers/furlongs.

Cheers,

Carl Fogel

and off course the rare RBT blooper where poster goes on at great
lenght on how his chains using cow fat go on for thousands of uh
kilometer miles or 26" 700c kilometer miles or....

seriously, before Magellan, the Cateye Enduro would hit mile markers
painted on Summerlin Road's bike path from San Carlos Blvd into Fort
Meyrs, FL: maintaining consistant pressures from initial adjustment.
On the mark-one mile/on the mark-two miles... 3 miles worth before the
cateye missed the mark by 4-5 inches.
5 pounds +/- of 75-80 would miss the mark by not more than an inch at
one mile using Conti TT.

I question mounting a bike spec GPS on the bars. a 76sx in a rack or
backpack does it without excessive exposure to accidental damage or
theft or vandalism bar mounted. Bar mounting maybe understood as a
sales gimmick not a useable fucntion.

PYA

On Sun, 18 Jul 2010 13:59:37 -0700 (PDT), kolldata
wrote:

and off course the rare RBT blooper where poster goes on at great
lenght on how his chains using cow fat go on for thousands of uh
kilometer miles or 26" 700c kilometer miles or....

.... and then crashes, but his chain survives because he was wearing a
h*lm*t

well, the really cool deal is the prob is usually simple oversight.
The normally intelligent used hasnot correctly discussed the issue
with his Cateye and not rechecked due to other more p[ressing problems
of life death taxes car payments due dates....
leading to an embarassing and hehehhe whoha.
the Cateye I treasure insists the tires are 26" when the tires are
not. 2-3 entries before Cateye gets the word.

On Jul 15, 3:49*pm, wrote:

I hope that someone will tell us how a GPS like the Garmin handles
distance with elevation change.

That's pretty simple. It measures the distance just as it would be
measured on a map
(NOT including a relief map).

DR

In article ,
Phil W Lee wrote:

Dan O considered Sun, 18 Jul 2010 08:47:23
-0700 (PDT) the perfect time to write:

On Jul 18, 2:57 am, James wrote:
On Jul 18, 5:31 pm, Michael Press wrote:

In article ,
Jobst Brandt wrote:

For precision, you need a road with accurate mile markers, as we have
on west coast state route CA 1 from Pigeon Point light house to Santa
Cruz that were placed there years ago to monitor driving speeds from
aircraft. These twelve inch wide white epoxy strips enable a
bicyclist to adjust speedometer calibration numbers to hit right on
the mile for a mile... if you care.

You also confuse precision and accuracy.
They are not synonyms. This was covered
in our freshman laboratories. I always
assumed other schools taught this as well.
Apparently not.

A web link might help..

http://en.wikipedia.org/wiki/Accuracy_and_precision

"In the fields of engineering, industry and statistics, the accuracy
of a measurement system is the degree of closeness of measurements of
a quantity to its actual (true) value. The precision of a measurement
system, also called reproducibility or repeatability, is the degree to
which repeated measurements under unchanged conditions show the same
results.[1] Although the two words can be synonymous in colloquial
use, they are deliberately contrasted in the context of the scientific
method."

So if you "hit right on the mile for a mile", you were accurate, but
not necessarily precise unless you could repeat the feat, time and
time again. Correct?

Regards,
James.

Hmm,.. I don't think so (of course, I am not formally educated or
anything like that, but... ) I think precision is simply the how
close your measurements are (they will never be "spot on" - sorry :-),
and accuracy is more about the correctness and reliability.

To give an extreme example, if someone asks "where are we" and the
answer is give as "planet earth", it is completely accurate, but so
imprecise as to be useless.

Wrong. The accuracy is ~8000 mile.
The precision is indeterminate.

--
Michael Press