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[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 

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