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Re: [ST] GPS as a Speedometer vs. Bicycle Computer



Are we RIDING MOTORCYCLES or attempting to send another rover to Mars???
Jeeziz! Some of you astute fellows obviously need to take a ride....you're
suffering from TooMuchFreeTimeOffTheBike...


Denis Liakos
Try and find me
got the eMap,
 use paper maps more often
- ----- Original Message ----- 
From: "Enrico Schuerrer" <enrico@xxxxxx>
To: <ST@xxxxxxxxxxxxxx>
Sent: Sunday, March 14, 2004 4:26 PM
Subject: Re: [ST] GPS as a Speedometer vs. Bicycle Computer


> Sorry to correct you - the bicycle computer depends on the rpm of the
front
> wheel, which will change the diameter in relation to the speed. So the
error
> is - depends on your calibration - around 5 % in minimum.
>
> The system immanent error of GPS in speed measuring is around 1%.
>
> So compare - 5% vs. 1% - which measuring will be more accurate? :-)
>
> Enrico
> --
> '00 ST 'BlueBelle'
> Vienna, Austria, Europe
>
> ----- Original Message -----
> From: <wcooper1@xxxxxxxxxxx>
> To: "digest ST" <ST@xxxxxxxxxxxxxx>
> Sent: Sunday, March 14, 2004 10:10 PM
> Subject: [ST] GPS as a Speedometer vs. Bicycle Computer
>
>
> > copied from the v strom list.....
> >
> > GPS as a Speedometer vs. Bicycle Computer
> >
> > A $15 Bicycle Computer is more accurate than a $500 GPS.
> >
> > If all you need is accurate speed and distance information, a bike
> computer will
> > give you more accurate information than any GPS. A bike computer has
less
> error than
> > the GPS system, so when properly calibrated, it gives superior results.
> >
> > How can this be true? A standard consumer GPS unit is limited in
accuracy
> by the
> > errors in the system. In most of the world, a handheld GPS is only
> accurate to +/-
> > 25 Metres with 95% confidence. If you will accept 50% confidence +/- 15
> Metres is
> > a valid number to use. This means that the GPS will tell you that you
have
> moved
> > 1 KM (1000 Metres) when the error on each end of the measurement could
> make that
> > 1000 Metres either 985 or 1015 metres, that is an error of 3.0% over one
> Kilometre.
> > As the distance increases the error becomes a smaller percentage, at one
> Mile the
> > error is 1.86%, at 10 KM the error is only 0.3%
> >
> > In the US newer GPS units can use the Wide Area Augmentation System
(WAAS)
> to get
> > +/- 3M accuracy at 95%. The error is now only 0.6% in a Kilometre.
> >
> > Certainly no $15 bike computer can be more accurate than that? But they
> are. A bike
> > computer counts wheel revolutions; it can never be off by more than 1
> revolution.
> > On the V-Strom the front wheel rolls about 2M each revolution (2088mm on
> my bike).
> > Maximum error in one Kilometre is thus less than 4M, or 0.4% this is
> better than
> > the 0.6% that a WASS enabled GPS unit can do. My Simga Sport gives
> reliable, repeatable
> > readings that are accurate within .01 KM (10M) over a surveyed 5KM.
> >
> > Another problem with GPS distance is that GPS units calculate distance
> from point
> > to point at a set rate. The NMEA standard is a 1 second update between
> updates. Each
> > of the positions is subject to the +/- error. This is not a problem for
> ships at
> > sea or aircraft, since they tend to move in straight lines. A vehicle
> seldom runs
> > a straight line for any length of time. At 100 KPH (62.15 MPH) your bike
> travels
> > 27.77M each second (91.15 feet). The best a GPS can do is to add up
these
> segments
> > to get the total distance traveled. The error is small but it exists.
> >
> > A bike computer measures distance every revolution of the wheel. At 100
> KPH the bike
> > computer has 13.889 data points each second compared to two data points
> for the GPS.
> > The bike computer will always give more accurate distance measurements,
> the more
> > changes in direction there are the better the bike computer does
compared
> to the
> > GPS.
> >
> > Garmin lists the accuracy of speed measurement as 0.05 Metres/Second "at
> steady state"
> > speed. This is because the GPS is using the average distance between
> points to calculate
> > speed. The steadier the speed and the straighter the course the more
> accurate the
> > GPS becomes, up to the limit of error. 0.05 M/S = 0.1852 KPH = 0.115078
> MPH so at
> > 100 KPH on a straight road your speed is between 99.81 KPH and 100.18
KPH
> (0.18%
> > error)
> >
> > A bike computer that can display speed to 300 KPH must be able to sample
> more than
> > 40 times per second. That makes the bike computer have a 0.002% error.
The
> bike computer
> > is an order of magnitude more accurate than the GPS for speed.
> >
> >
> >
> > --
> > thanks//////coop
> >
> > No trees were killed in the sending of this message. However a large
> number of electrons were terribly inconvenienced...
> >
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> *
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>
>
>
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