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

["Enrico Schuerrer" <enrico@xxxxxx>]

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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